Meaning of the word twin

English[edit]

Alternative forms[edit]

• twynne (obsolete)

Pronunciation[edit]

• enPR: twĭn, IPA^(key): /twɪn/, [tw̥ɪn]
• Rhymes: -ɪn

Etymology 1[edit]

PIE word
*dwóh₁

From Middle English twinne, twynne, from Old English ġetwin, ġetwinn (“twin, multiple”, noun) and twinn (“twin, two-fold, double, two by two”, adjective), from Proto-Germanic *twinjaz, *twinaz (“two each”), from Proto-Indo-European *dwino- (“twin”), from *dwóh₁ (“two”). Cognate with Scots twyn (“twin”), Dutch tweeling (“twin”), German Zwilling (“twin”), Swedish tvilling (“twin”), Faroese tvinnur (“a double set”), Icelandic tvenna (“duo, pair”), Lithuanian dvynys (“twin”), Russian двойня (dvojnja, “twin”).

Noun[edit]

twin (plural twins)

1. Either of two people (or, less commonly, animals) who shared the same uterus at the same time; one who was born at the same birth as a sibling.
2. Either of two similar or closely related objects, entities etc.
3. A room in a hotel, guesthouse, etc. with two beds; a twin room.
4. (US) A twin size mattress or a bed designed for such a mattress.
5. (aviation) A two-engine aircraft.
6. (crystallography) A twin crystal.

Synonyms[edit]

• twindle, twinling, doublet (in the sense of twins and triplets)

Derived terms[edit]

Translations[edit]

See also[edit]

• twyndyllyng
• (hotel room): single, double
• twain

Verb[edit]

twin (third-person singular simple present twins, present participle twinning, simple past and past participle twinned)

1. (transitive, obsolete outside Scotland) To separate, divide.
2. (intransitive, obsolete outside Scotland) To split, part; to go away, depart.
3. (usually in the passive) To join, unite; to form links between (now especially of two places in different countries); to pair with.

   Reading, the English town, is twinned with Clonmel in Ireland.

   Coventry twinned with Dresden as an act of peace and reconciliation, both cities having been heavily bombed during the war.

   • Still we moved / Together, twinned, as horse’s ear and eye.

   • 2006, Aruna D’Souza; Tom McDonough; Tom Mc Donough, The Invisible Flâneuse?: Gender, Public Space, and Visual Culture in Nineteenth-century Paris, Manchester University Press, →ISBN, page 60:

     Yet, Manet heightens its effect by performing the clever ocular trick of practically twinning her with her pictorial counterpart, Madame Lejosne.

4. (intransitive) To give birth to twins.

   • 1874, Thomas Hardy, Far from the Madding Crowd:

     “I’ve run to tell ye,” said the junior shepherd, supporting his exhausted youthful frame against the doorpost, “that you must come directly. Two more ewes have twinned — that’s what’s the matter, Shepherd Oak.”

   • 1992, George Christopher Williams, Natural Selection: Domains, Levels, and Challenges, Oxford University Press on Demand, →ISBN, page 79:

     Twinning today is abnormal in all higher primates, and possibly adaptive only in the one species with an obstetrical technology that permits a high rate of survival of twins.

5. (transitive) To be, or be like, a twin to (someone else); to match in some way.

   • 2009, Kathryn Bond Stockton, The Queer Child, or Growing Sideways in the Twentieth Century, Duke University Press, →ISBN, page 111:

     The invert child and her innocent child are together lost children, twinning each other despite their distinctions.

   • 2017, Christina Crosby, A Body, Undone: Living On After Great Pain, NYU Press, →ISBN, page 111:

     I was awake to the horror of our twinning each other in paralysis, and feared that we would soon talk only about the daily, intimate care serious paralysis demands […]

6. (intransitive) To be, or be like, a pair of twins (for example, to dress identically); to be paired or suited.

   • 2018, Vivian Blue, Your Heart Is Mine: A Criminal Romance, Sullivan Group Publishing, →ISBN:

     “I see you got us twinning today.” Byrd kissed Lay quickly. “I wanted you to dress like me today,” he admitted.

   • 2019, Lisa Mullarkey, Paisley, ABDO, →ISBN, page 17:

     “We’re totally twinning today!” said Zoey. “You sure are,” said Chef Piper. “I whipped up a special dessert for tonight.”

See also[edit]

• sister city

Etymology 2[edit]

From Middle English *twin, *twyn, from Old English twin, twinn (“twin; double”, adjective), from Proto-Germanic *twīhnaz (“occurring in a pair; twofold; double”), from Proto-Indo-European *dwóh₁ (“two”). Cognate with Icelandic tvennur (“double”), Gothic 𐍄𐍅𐌴𐌹𐌷𐌽𐌰𐌹 (tweihnai, “two each”).

Adjective[edit]

twin (not comparable)

1. Double; dual; occurring as a matching pair.

   twin beds, twin socks

2. Forming a pair of twins.

   the twin boys

Synonyms[edit]

• (forming a matched pair): twofold; see also Thesaurus:dual

Derived terms[edit]

Translations[edit]

Further reading[edit]

• twin in Webster’s Revised Unabridged Dictionary, G. & C. Merriam, 1913
• “twin”, in The Century Dictionary […], New York, N.Y.: The Century Co., 1911, →OCLC.
• twin at OneLook Dictionary Search

Anagrams[edit]

• Wint

Middle English[edit]

Noun[edit]

twin

1. Alternative form of twyn

Old English[edit]

Pronunciation[edit]

• IPA^(key): /twin/

Adjective[edit]

twin

1. Alternative form of twinn

Declension[edit]

Recent Examples on the Web

In a video taken at the tour opener, both musicians could be seen on a screen in all-white attire joined by Blue Ivy and twins Rumi and Sir for what appeared to be a vow renewal ceremony.


—Melody Chiu, Peoplemag, 4 Apr. 2023




The missing girl’s parents, Kate and Gerry McCann, along with their three children – Madeleine and twins Sean and Amelie – were on vacation in Praia da Luz, Portugal, when Madeleine was taken from her bed on May 3, 2007.


—Audrey Conklin, Fox News, 4 Apr. 2023




The couple said their tiny twins faced many challenges, including difficulty breathing because of undeveloped lungs, digestive problems and bone fractures.


—Cathy Free, Washington Post, 3 Apr. 2023




The original movie gave Jeremy Irons dual roles as both the main characters, but the TV version has gender-swapped the lead, so the series will star Rachel Weisz — who has experience playing twins before in 2005’s Constantine.


—Brendan Morrow, The Week, 1 Apr. 2023




In 2022, twins Billie and Jade, the children of Dr Albert Wesker, move to New Raccoon City, a community designed by the Umbrella Corporation.


—Amy Mackelden, ELLE, 31 Mar. 2023




Playing opposite herself required a learning curve, said the pair, who described the technical process of capturing each twin’s perspective when in a scene together.


—Jackie Strause, The Hollywood Reporter, 29 Mar. 2023




She is trailed by Haley and Hanna Cavinder, twins who transferred from Fresno State to Miami and led their team to the Elite 8 for the first time in program history.


—Lia Assimakopoulos, Dallas News, 29 Mar. 2023




Based on those experiments, the Reefy team has created a computer model that works a bit like a digital twin, but for the ocean.


—WIRED, 28 Mar. 2023

The transom is home to an electro-hydraulic bathing platform and a garage with space for a Williams SportJet 435 tender, while the cockpit is equipped with twin sun pads, lounge seating, and two coffee tables.


—Rachel Cormack, Robb Report, 24 Mar. 2023




Ras Al Khaimah is also a fantastic location for a twin-center vacation with Dubai.


—Michele Robson, Forbes, 23 Mar. 2023




For those who dislike the hassle of a zip closure, the brand’s twin latch locks will be a breath of fresh air.


—Sophie Dodd, Travel + Leisure, 23 Mar. 2023




Miami twin basketball players Haley and Hanna Cavinder are social media sensations who have cashed in on their online fame.


—John Marshall, The Christian Science Monitor, 23 Mar. 2023




Harman Kardon and Alexa available The interior continues the family resemblance: Dodge’s twin-rhombus slashes adorn the steering wheel and the 10.25-in touch screen has the latest version of Stellantis’s excellent Uconnect infotainment system.


—Mark Phelan, Detroit Free Press, 22 Mar. 2023




George had twin surgeries in his right and left shoulders in spring 2019 while with Oklahoma City.


—Beth Harris, BostonGlobe.com, 22 Mar. 2023




The principal element in this metamorphosis is a new twin-cam, sixteen-valve, four-cylinder engine, producing 123 horsepower and 120 pounds-feet of torque.


—Csaba Csere, Car and Driver, 21 Mar. 2023




The twin economic challenges pose a dilemma for the Federal Reserve because its strongest tool, the benchmark interest rate, is a key cause of the financial emergency but the primary solution for high prices.


—Max Zahn, ABC News, 20 Mar. 2023

Shoppers can choose from seven colors, including neutral hues like white and taupe, and a pastel shade called spa blue, all of which are available in sizes twin through California king.


—Toni Sutton, Peoplemag, 21 Mar. 2023




Shoppers can choose from several colors, including navy and charcoal, all of which are available in sizes twin through California king.


—Amy Schulman, Peoplemag, 26 Feb. 2023




Pets can twin with their human parents in polos, tees, knitwear and coats ($250 to $850) in playful strawberry and heart prints, classic monogram patterns and more.


—Danielle Directo-meston, The Hollywood Reporter, 27 June 2022




Kim Kardashian took a break from Skims bodysuits this Easter to twin with her kids.


—Glamour, 25 Apr. 2022




Now researchers are shooting for the loftiest goal: to twin the brain.


—Grace Browne, Wired, 15 Feb. 2022




For instance, healthcare professionals can one day digitally twin a patient’s body to test the impact of a treatment.


—Sayantan Dasgupta, Forbes, 7 Dec. 2021




The always stylish couple loves to keep their outfits coordinated and during a fashion fitting over the weekend, Union, 48, and Wade, 39, decided to twin in identical patterned pants.


—Kaitlyn Frey, PEOPLE.com, 25 Oct. 2021




If ‘1917’ doesn’t twin the top Oscar, ‘Once Upon a Time … in Hollywood’ will.


—Michael Phillips, chicagotribune.com, 25 Nov. 2020


See More

Twins are two offspring produced by the same pregnancy.^[1] Twins can be either monozygotic (‘identical’), meaning that they develop from one zygote, which splits and forms two embryos, or dizygotic (‘non-identical’ or ‘fraternal’), meaning that each twin develops from a separate egg and each egg is fertilized by its own sperm cell.^[2] Since identical twins develop from one zygote, they will share the same sex, while fraternal twins may or may not. In rare cases twins can have the same mother and different fathers (heteropaternal superfecundation).

In contrast, a fetus that develops alone in the womb (the much more common case, in humans) is called a singleton, and the general term for one offspring of a multiple birth is a multiple.^[3] Unrelated look-alikes whose resemblance parallels that of twins are referred to as doppelgängers.^[4]

Statistics[edit]

The human twin birth rate in the United States rose 76% from 1980 through 2009, from 9.4 to 16.7 twin sets (18.8 to 33.3 twins) per 1,000 births.^[5] The Yoruba people have the highest rate of twinning in the world, at 45–50 twin sets (90–100 twins) per 1,000 live births,^[6][7][8] possibly because of high consumption of a specific type of yam containing a natural phytoestrogen which may stimulate the ovaries to release an egg from each side.^[9][10] In Central Africa, there are 18–30 twin sets (or 36–60 twins) per 1,000 live births.^[11] In South America, South Asia (India, Pakistan, Bangladesh, Nepal), and Southeast Asia, the lowest rates are found; only 6 to 9 twin sets per 1,000 live births. North America and Europe have intermediate rates of 9 to 16 twin sets per 1,000 live births.^[11]

Multiple pregnancies are much less likely to carry to full term than single births, with twin pregnancies lasting on average 37 weeks, three weeks less than full term.^[12] Women who have a family history of fraternal twins have a higher chance of producing fraternal twins themselves, as there is a genetically linked tendency to hyper-ovulate. There is no known genetic link for identical twinning.^[13] Other factors that increase the odds of having fraternal twins include maternal age, fertility drugs and other fertility treatments, nutrition, and prior births.^[14] Some women intentionally turn to fertility drugs in order to conceive twins.^{[15][better source needed]}

Types and zygosity[edit]

The vast majority of twins are either dizygotic (fraternal) or monozygotic (identical). Less common variants are discussed further down the article.

Fraternal twins can be any of the following:

• Female–female twins: Sometimes called sororal twins (25%).
• Male–male twins: Sometimes called fraternal twins (25%).
• Female-male twins: This is the most common pairing (50%), encompassing both «female-male» (25%) and «male-female» (25%) twins.

Among non-twin births, male singletons are slightly (about five percent) more common than female singletons. The rates for singletons vary slightly by country. For example, the sex ratio of birth in the US is 1.05 males/female,^[16] while it is 1.07 males/female in Italy.^[17] However, males are also more susceptible than females to die in utero, and since the death rate in utero is higher for twins, it leads to female twins being more common than male twins.^[18]

Zygosity is the degree of identity in the genome of twins.

Common name	Scientific name	Zygosity	Development	Occurrence	Identification	Health	Other
Identical	Monozygotic	x	x	x	x	x	x
Fraternal	Dizygotic	x	x	x	x	x	x
Half-identical	Sesquizygotic[19]	x	x	x	x	x	x
Mirror image	x	x	x	x	x	x	x
Mixed chromosome	x	x	x	x	x	x	x
Superfecundation	x	x	Eggs are fertilized during different acts of intercourse	x	x	x	Usage is practically equivalent with heteropaternal superfecundation, which occurs when two different males father fraternal twins, because though superfecundation by the same father is thought to be a common occurrence, it can only be proven to have occurred with multiple fathers.
Superfetation	x	x	A female gets pregnant again while already pregnant, resulting in multiple fetuses at differing developmental stages	x	x	x	x
Parasitic twin	x	x	x	x	x	By definition only healthy fully formed fetus	x
Vanishing twin	Twin resorption, twin embolisation syndrome	x	x	Up to 1 of every 8 multifetus pregnancies	x	By definition only healthy fully formed fetus	Chimerism, mosaicism
Polar body	x	x	x	x	x	x	x
Conjoined twin	x	x	x	x	x	Ranges from normal to compromised	x

Dizygotic (fraternal) twins[edit]

Dizygotic (DZ) or fraternal twins (also referred to as «non-identical twins», «dissimilar twins», «biovular twins», and, informally in the case of females, «sororal twins») usually occur when two fertilized eggs are implanted in the uterus wall at the same time. When two eggs are independently fertilized by two different sperm cells, fraternal twins result. The two eggs, or ova, form two zygotes, hence the terms dizygotic and biovular. Fraternal twins are, essentially, two ordinary siblings who happen to develop in the womb together and who are born at the same time, since they arise from two separate eggs fertilized by two separate sperm, just like ordinary siblings. This is the most common type of twin.^[20]

Dizygotic twins, like any other siblings, will practically always have different sequences on each chromosome, due to chromosomal crossover during meiosis. Dizygotic twins share only 50 percent of each other’s genes, which resemble amongst siblings that are conceived and born at different times. Like any other siblings, dizygotic twins may look similar, particularly as they are the same age. However, dizygotic twins may also look very different from each other (for example, be of opposite sexes).

Studies show that there is a genetic proclivity for dizygotic twinning. However, it is only the mother who has any effect on the chances of having such twins; there is no known mechanism for a father to cause the release of more than one ovum. Dizygotic twinning ranges from six per thousand births in Japan (similar to the rate of monozygotic twins) to 14 and more per thousand in some African countries.^[21]

Dizygotic twins are also more common for older mothers, with twinning rates doubling in mothers over the age of 35.^[22] With the advent of technologies and techniques to assist women in getting pregnant, the rate of fraternals has increased markedly.^{[citation needed]}

Monozygotic (identical) twins[edit]

Monozygotic (MZ) or identical twins occur when a single egg is fertilized to form one zygote (hence, «monozygotic») which then divides into two separate embryos. The chances of having identical twins is relatively rare — around 3 or 4 in every 1,000 births.^[23]

Mechanism[edit]

Regarding spontaneous or natural monozygotic twinning, a recent theory proposes that monozygotic twins are probably formed when a blastocyst contains two inner cell masses (ICM), each of which will lead to a separate fetus, rather than by the embryo splitting while hatching from the zona pellucida (the gelatinous protective coating around the blastocyst).^[24]

Monozygotic twins may also be created artificially by embryo splitting. It can be used as an expansion of in vitro fertilization (IVF) to increase the number of available embryos for embryo transfer.^[25]

Incidence[edit]

Monozygotic twinning occurs in birthing at a rate of about 3 in every 1000 deliveries worldwide (about 0.3% of the world population).^[26]

The likelihood of a single fertilization resulting in monozygotic twins is uniformly distributed in all populations around the world.^[22] This is in marked contrast to dizygotic twinning, which ranges from about six per thousand births in Japan (almost similar to the rate of identical twins, which is around 4–5) to 15 and more per thousand in some parts of India^[27] and up to over 20 in some Central African countries.^[11] The exact cause for the splitting of a zygote or embryo is unknown.

IVF techniques are more likely to create dizygotic twins. For IVF deliveries, there are nearly 21 pairs of twins for every 1,000.^[28]

Genetic and epigenetic similarity[edit]

Monozygotic twins are genetically nearly identical and they are the same chromosomal sex unless there has been a mutation during development. The children of monozygotic twins test genetically as half-siblings (or full siblings, if a pair of monozygotic twins reproduces with another pair or with the same person), rather than first cousins. Identical twins do not have the same fingerprints however, because even within the confines of the womb, the fetuses touch different parts of their environment, giving rise to small variations in their corresponding prints and thus making them unique.^[29]

Monozygotic twins always have the same genotype. Normally due to an environmental factor or the deactivation of different X chromosomes in female monozygotic twins, and in some extremely rare cases, due to aneuploidy, twins may express different sexual phenotypes, normally from an XXY Klinefelter syndrome zygote splitting unevenly.^[30][31][32]

Monozygotic twins, although genetically very similar, are not genetically exactly the same. The DNA in white blood cells of 66 pairs of monozygotic twins was analyzed for 506,786 single-nucleotide polymorphisms known to occur in human populations. Polymorphisms appeared in 2 of the 33 million comparisons, leading the researchers to extrapolate that the blood cells of monozygotic twins may have on the order of one DNA-sequence difference for every 12 million nucleotides, which would imply hundreds of differences across the entire genome.^[33] The mutations producing the differences detected in this study would have occurred during embryonic cell-division (after the point of fertilization). If they occur early in fetal development, they will be present in a very large proportion of body cells.

Another cause of difference between monozygotic twins is epigenetic modification, caused by differing environmental influences throughout their lives. Epigenetics refers to the level of activity of any particular gene. A gene may become switched on, switched off, or could become partially switched on or off in an individual. This epigenetic modification is triggered by environmental events. Monozygotic twins can have markedly different epigenetic profiles. A study of 80 pairs of monozygotic twins ranging in age from three to 74 showed that the youngest twins have relatively few epigenetic differences. The number of epigenetic differences increases with age. Fifty-year-old twins had over three times the epigenetic difference of three-year-old twins. Twins who had spent their lives apart (such as those adopted by two different sets of parents at birth) had the greatest difference.^[34] However, certain characteristics become more alike as twins age, such as IQ and personality.^[35][36][37]

In January 2021, new research from a team of researchers in Iceland was published in the journal Nature Genetics suggesting that identical twins may not be quite as identical as previously thought.^{[full citation needed]} The four-year study of monozygotic (identical) twins and their extended families revealed that these twins have genetic differences that begin in the early stages of embryonic development.^[38]

Polar body and semi-identical twins[edit]

A 1981 study of a deceased triploid XXX twin fetus without a heart showed that although its fetal development suggested that it was an identical twin, as it shared a placenta with its healthy twin, tests revealed that it was probably a polar body twin. The authors were unable to predict whether a healthy fetus could result from a polar body twinning.^[39] However, a study in 2012 found that it is possible for a polar body to result in a healthy fetus.^[40]

In 2003, a study argued that many cases of triploidy arise from sesquizygotic (semi-identical) twinning.^[41]

Degree of separation[edit]

The degree of separation of the twins in utero depends on if and when they split into two zygotes. Dizygotic twins were always two zygotes. Monozygotic twins split into two zygotes at some time very early in the pregnancy. The timing of this separation determines the chorionicity (the number of placentae) and amniocity (the number of sacs) of the pregnancy. Dichorionic twins either never divided (i.e.: were dizygotic) or they divided within the first 4 days. Monoamnionic twins divide after the first week.

In very rare cases, twins become conjoined twins. Non-conjoined monozygotic twins form up to day 14 of embryonic development, but when twinning occurs after 14 days, the twins will likely be conjoined.^[42] Furthermore, there can be various degrees of shared environment of twins in the womb, potentially leading to pregnancy complications.

It is a common misconception that two placentas automatically implies dizygotic twins, but if monozygotic twins separate early enough, the arrangement of sacs and placentas in utero is in fact indistinguishable from that of dizygotic twins.

Type	Description	Day
Dichorionic-Diamniotic	Normally, twins have two separate (di- being a numerical prefix for two) chorions and amniotic sacs, termed Dichorionic-Diamniotic or «DiDi». It occurs in almost all cases of dizygotic twins (except in very rare cases of fusion between their blastocysts[43]) and in 18–36%[44] (or around 25%[43]) of monozygotic (identical) twins. DiDi twins have the lowest mortality risk at about 9 percent, although that is still significantly higher than that of singletons.[45]	Dichorionic-Diamniotic twins form when splitting takes place by the third day after fertilization.[43]
Monochorionic-Diamniotic	Monochorionic twins share the same placenta. Monochorionic twins generally have two amniotic sacs (called Monochorionic-Diamniotic «MoDi»), which occurs in 60–70% of the pregnancies with monozygotic twins,[44] and in 0.3% of all pregnancies.[46] Monochorionic-Diamniotic twins are almost always monozygotic, with a few exceptions where the blastocysts have fused.[43] Monochorionic twins share the same placenta, and thus have a risk of twin-to-twin transfusion syndrome.	Days 4-8
Monochorionic-Monoamniotic	Monochorionic twins share the same amnion in 1–2% of monozygotic twin pregnancies.[44] Monoamniotic twins are always monozygotic.[47] The survival rate for monoamniotic twins is somewhere between 50%[47] to 60%.[48] Monoamniotic twins, as with diamniotic monochorionic twins, have a risk of twin-to-twin transfusion syndrome. Also, the two umbilical cords have an increased chance of being tangled around the babies. Because of this, there is an increased chance that the newborns may be miscarried or suffer from cerebral palsy due to lack of oxygen.	Monoamniotic twins occur when the split takes place after the ninth day after fertilization.[43]
Conjoined twins	When the division of the developing zygote into 2 embryos occurs, 99% of the time it is within 8 days of fertilization. Mortality is highest for conjoined twins due to the many complications resulting from shared organs.	If the division of the zygote occurs later than the 12 days then conjoined twins are usually the result.

• 

  Dichorionic-diamniotic twins at 8 weeks and 5 days since co-incubation as part of IVF. The twin at left in the image is shown in the sagittal plane with the head pointing towards upper left. The twin at right in the image is shown in the coronal plane with the head pointing rightwards.

Demographics[edit]

A 2006 study has found that insulin-like growth factor present in dairy products may increase the chance of dizygotic twinning. Specifically, the study found that vegan mothers (who exclude dairy from their diets) are one-fifth as likely to have twins as vegetarian or omnivore mothers, and concluded that «Genotypes favoring elevated IGF and diets including dairy products, especially in areas where growth hormone is given to cattle, appear to enhance the chances of multiple pregnancies due to ovarian stimulation.»^[49]

From 1980 to 1997, the number of twin births in the United States rose 52%.^[50] This rise can at least partly be attributed to the increasing popularity of fertility drugs and procedures such as IVF, which result in multiple births more frequently than unassisted fertilizations do. It may also be linked to the increase of growth hormones in food.^[49]

Ethnicity[edit]

About 1 in 90 human births (1.1%) results from a twin pregnancy.^[51] The rate of dizygotic twinning varies greatly among ethnic groups, ranging as high as about 45 per 1000 births (4.5%) for the Yoruba to 10% for Linha São Pedro, a tiny Brazilian settlement which belongs to the city of Cândido Godói.^[52] In Cândido Godói, one in five pregnancies has resulted in twins.^[53] The Argentine historian Jorge Camarasa has put forward the theory that experiments of the Nazi doctor Josef Mengele could be responsible for the high ratio of twins in the area. His theory was rejected by Brazilian scientists who had studied twins living in Linha São Pedro; they suggested genetic factors within that community as a more likely explanation.^[54] A high twinning rate has also been observed in other places of the world, including:

• Igbo-Ora in Nigeria^[55]
• Kodinhi, located in Kerala, India^[56]
• Mohammadpur Umri, located in Uttar Pradesh, India^[57][58][59]

The widespread use of fertility drugs causing hyperovulation (stimulated release of multiple eggs by the mother) has caused what some call an «epidemic of multiple births». In 2001, for the first time ever in the US, the twinning rate exceeded 3% of all births. Nevertheless, the rate of monozygotic twins remains at about 1 in 333 across the globe.

In a study on the maternity records of 5750 Hausa women living in the Savannah zone of Nigeria, there were 40 twins and 2 triplets per 1000 births. Twenty-six percent of twins were monozygotic. The incidence of multiple births, which was about five times higher than that observed in any western population, was significantly lower than that of other ethnic groups, who live in the hot and humid climate of the southern part of the country. The incidence of multiple births was related to maternal age but did not bear any association to the climate or prevalence of malaria.^[60][61]

Twins are more common in people of African descent.^[62]

Predisposing factors[edit]

The predisposing factors of monozygotic twinning are unknown.

Dizygotic twin pregnancies are slightly more likely when the following factors are present in the woman:

• She is of West African descent (especially Yoruba)
• She is between the age of 30 and 40 years
• She is greater than average height and weight
• She has had several previous pregnancies.

Women undergoing certain fertility treatments may have a greater chance of dizygotic multiple births. In the United States it has been estimated that by 2011 36% of twin births resulted from conception by assisted reproductive technology.^[63]

The risk of twin birth can vary depending on what types of fertility treatments are used. With in vitro fertilisation (IVF), this is primarily due to the insertion of multiple embryos into the uterus. Ovarian hyperstimulation without IVF has a very high risk of multiple birth. Reversal of anovulation with clomifene (trade names including Clomid) has a relatively less but yet significant risk of multiple pregnancy.

Delivery interval[edit]

A 15-year German study^[64] of 8,220 vaginally delivered twins (that is, 4,110 pregnancies) in Hesse yielded a mean delivery time interval of 13.5 minutes.^[65] The delivery interval between the twins was measured as follows:

• Within 15 minutes: 75.8%
• 16–30 minutes: 16.4%
• 31–45 minutes: 4.3%
• 46–60 minutes: 1.7%
• Over 60 minutes: 1.8%

The study stated that the occurrence of complications «was found to be more likely with increasing twin-to-twin delivery time interval» and suggested that the interval be kept short, though it noted that the study did not examine causes of complications and did not control for factors such as the level of experience of the obstetrician, the wish of the women giving birth, or the «management strategies» of the procedure of delivering the second twin.

There have also been cases in which twins are born a number of days apart. Possibly the worldwide record for the duration of the time gap between the first and the second delivery was the birth of twins 97 days apart in Cologne, Germany, the first of which was born on November 17, 2018.^[66]

Complications during pregnancy[edit]

Vanishing twins[edit]

Researchers suspect that as many as 1 in 8 pregnancies start out as multiples, but only a single fetus is brought to full term, because the other fetus has died very early in the pregnancy and has not been detected or recorded.^[67] Early obstetric ultrasonography exams sometimes reveal an «extra» fetus, which fails to develop and instead disintegrates and vanishes in the uterus. There are several reasons for the «vanishing» fetus, including it being embodied or absorbed by the other fetus, placenta or the mother. This is known as vanishing twin syndrome. Also, in an unknown proportion of cases, two zygotes may fuse soon after fertilization, resulting in a single chimeric embryo, and, later, fetus.

Conjoined twins[edit]

Conjoined twins (or the once-commonly used term «siamese») are monozygotic twins whose bodies are joined during pregnancy. This occurs when the zygote starts to split after day 12^[43] following fertilization and fails to separate completely. This condition occurs in about 1 in 50,000 human pregnancies.
Most conjoined twins are now evaluated for surgery to attempt to separate them into separate functional bodies. The degree of difficulty rises if a vital organ or structure is shared between twins, such as the brain, heart, liver or lungs.

Chimerism[edit]

A chimera is an ordinary person or animal except that some of their parts actually came from their twin or from the mother. A chimera may arise either from monozygotic twin fetuses (where it would be impossible to detect), or from dizygotic fetuses, which can be identified by chromosomal comparisons from various parts of the body. The number of cells derived from each fetus can vary from one part of the body to another, and often leads to characteristic mosaicism skin coloration in human chimeras. A chimera may be intersex, composed of cells from a male twin and a female twin. In one case DNA tests determined that a woman, Lydia Fairchild, mystifyingly, was not the mother of two of her three children; she was found to be a chimera, and the two children were conceived from eggs derived from cells of their mother’s twin.^[68]

Parasitic twins[edit]

Sometimes one twin fetus will fail to develop completely and continue to cause problems for its surviving twin. One fetus acts as a parasite towards the other.
Sometimes the parasitic twin becomes an almost indistinguishable part of the other, and sometimes this needs to be treated medically.

Partial molar twins[edit]

A very rare type of parasitic twinning is one where a single viable twin is endangered when the other zygote becomes cancerous, or «molar». This means that the molar zygote’s cellular division continues unchecked, resulting in a cancerous growth that overtakes the viable fetus. Typically, this results when one twin has either triploidy or complete paternal uniparental disomy, resulting in little or no fetus and a cancerous, overgrown placenta, resembling a bunch of grapes.

Miscarried twin[edit]

Occasionally, a woman will suffer a miscarriage early in pregnancy, yet the pregnancy will continue; one twin was miscarried but the other was able to be carried to term. This occurrence is similar to the vanishing twin syndrome, but typically occurs later, as the twin is not reabsorbed.

Low birth weight[edit]

It is very common for twins to be born at a low birth weight. More than half of twins are born weighing less than 5.5 pounds (2.5 kg), while the average birth weight of a healthy baby should be around 6–8 pounds (3–4 kg).^[69] This is largely due to the fact that twins are typically born premature. Premature birth and low birth weights, especially when under 3.5 pounds (1.6 kg), can increase the risk of several health-related issues, such as vision and hearing loss, mental disabilities, and cerebral palsy.^[70] There is an increased possibility of potential complications as the birth weight of the baby decreases.

Twin-to-twin transfusion syndrome[edit]

Monozygotic twins who share a placenta can develop twin-to-twin transfusion syndrome. This condition means that blood from one twin is being diverted into the other twin. One twin, the ‘donor’ twin, is small and anemic, the other, the ‘recipient’ twin, is large and polycythemic. The lives of both twins are endangered by this condition.

Stillbirths[edit]

Stillbirths occurs when a fetus dies after 20 weeks of gestation. There are two types of stillbirth, including intrauterine death and intrapartum death. Intrauterine death occurs when a baby dies during late pregnancy. Intrapartum death, which is more common, occurs when a baby dies while the mother is giving birth. The cause of stillbirth is often unknown, but the rate of babies who are stillborn is higher in twins and multiple births. Caesareans or inductions are advised after 38 weeks of pregnancy for twins, because the risk of stillbirth increases after this time.^[71]

Heterotopic pregnancy[edit]

Heterotopic pregnancy is an exceedingly rare type of dizygotic twinning in which one twin implants in the uterus as normal and the other remains in the fallopian tube as an ectopic pregnancy. Ectopic pregnancies must be resolved because they can be life-threatening to the mother. However, in most cases, the intrauterine pregnancy can be salvaged.^{[citation needed]}

Management of birth[edit]

For otherwise healthy twin pregnancies where both twins are head down a trial of vaginal delivery is recommended at between 37 and 38 weeks.^[72][73] Vaginal delivery in this case does not worsen the outcome for the infant as compared with Caesarean section.^[72] There is controversy on the best method of delivery where the first twin is head first and the second is not.^[72] When the first twin is not head down a caesarean section is often recommended.^[72] It is estimated that 75% of twin pregnancies in the United States were delivered by caesarean section in 2008.^[74] In comparison, the rate of caesarean section for all pregnancies in the general population varies between 14% and 40%.^[75] In twins that share the same placenta, delivery may be considered at 36 weeks.^[76] For twins who are born early, there is insufficient evidence for or against placing preterm stable twins in the same cot or incubator (co-bedding).^[77]

Human twin studies[edit]

Twin studies are utilized in an attempt to determine how much of a particular trait is attributable to either genetics or environmental influence. These studies compare monozygotic and dizygotic twins for medical, genetic, or psychological characteristics to try to isolate genetic influence from epigenetic and environmental influence. Twins that have been separated early in life and raised in separate households are especially sought-after for these studies, which have been used widely in the exploration of human nature. Classical twin studies are now being supplemented with molecular genetic studies which identify individual genes.

Unusual twinnings[edit]

Bi-paternal twins[edit]

This phenomenon is known as heteropaternal superfecundation. One 1992 study estimates that the frequency of heteropaternal superfecundation among dizygotic twins, whose parents were involved in paternity suits, was approximately 2.4%.^{[citation needed]}

Mixed twins[edit]

Dizygotic twins from biracial couples can sometimes be mixed twins, which exhibit differing ethnic and racial features. One such pairing was born in London in 1993 to a white mother and Caribbean father.^[78]

Monozygotic twins of different sexes[edit]

Among monozygotic twins, in extremely rare cases, twins have been born with different sexes (one male, one female).^[79] When monozygotic twins are born with different sexes it is because of chromosomal defects. The probability of this is so small that multiples having different sexes is universally accepted as a sound basis for in utero clinical determination that the multiples are not monozygotic.

Another abnormality that can result in monozygotic twins of different sexes is if the egg is fertilized by a male sperm but during cell division only the X chromosome is duplicated. This results in one normal male (XY) and one female with Turner syndrome (45,X).^[80] In these cases, although the twins did form from the same fertilized egg, it is incorrect to refer to them as genetically identical, since they have different karyotypes.

Semi-identical (sesquizygotic) twins[edit]

Monozygotic twins can develop differently, due to their genes being differently activated.^[81] More unusual are «semi-identical twins», also known as «sesquizygotic». As of 2019, only two cases have been reported.^[82][83] These «half-identical twins» are hypothesized to occur when an unfertilized egg cleaves into two identical attached ova, both of which are viable for fertilization. Both ova are then fertilized, each by a different sperm, and the coalesced zygotes undergo further cell duplications developing as a chimeric blastomere. If this blastomere then undergoes a twinning event, two embryos will be formed, with different paternal genetic information and identical maternal genetic information.

This results in a set of twins with identical gene sequence from the mother’s side, but different sequences from the father’s side. Cells in each fetus carry chromosomes from either sperm, resulting in chimeras. This form had been speculated until only recently being recorded.^[84][85][86]

In 2007, a study reported a case of a pair of living twins, which shared an identical set of maternal chromosomes, while each having a distinct set of paternal chromosomes, albeit from the same man, and thus they most likely share half of their father’s genetic makeup. The twins were both found to be chimeras. One was an intersex XX, and one a XY male. The exact mechanism of fertilization could not be determined but the study stated that it was unlikely to be a case of polar body twinning.^[84][87]

The likely genetic basis of semi-identical twins was reported in 2019 by Michael Gabbett and Nicholas Fisk. In their seminal publication, Gabbett, Fisk and colleagues documented a second case of sesquizygosis and presented molecular evidence of the phenomenon.^[82] The reported twins shared 100% of their maternal chromosomes and 78% of their paternal genomic information. The authors presented evidence that two sperm from the same man fertilized an ovum simultaneously. The chromosomes assorted themselves through heterogonesis to form three cell lines. The purely paternal cell line died out due to genomic imprinting lethality, while the other two cell lines, each consisting of the same maternal DNA but only 50% identical paternal DNA, formed a morula which subsequently split into twins.^[82][88]

Mirror image twins[edit]

Mirror image twins result when a fertilized egg splits later in the embryonic stage than normal timing, around day 9–12. This type of twinning could exhibit characteristics with reversed asymmetry, such as opposite dominant handedness, dental structure, or even organs (situs inversus).^[89] If the split occurs later than this time period, the twins risk being conjoined. There is no DNA-based zygosity test that can determine if twins are indeed mirror image.^[90] The term «mirror image» is used because the twins, when facing each other, appear as matching reflections.^[91]

Language development[edit]

There have been many studies highlighting the development of language in twins compared to single-born children. These studies have converged on the notion that there is a greater rate of delay in language development in twins compared to their single-born counterparts.^[92] The reasons for this phenomenon are still in question; however, cryptophasia was thought to be the major cause.^[93] Idioglossia is defined as a private language that is usually invented by young children, specifically twins. Another term to describe what some people call «twin talk» is cryptophasia where a language is developed by twins that only they can understand. The increased focused communication between two twins may isolate them from the social environment surrounding them. Idioglossia has been found to be a rare occurrence and the attention of scientists has shifted away from this idea. However, there are researchers and scientists that say cryptophasia or idioglossia is not a rare phenomenon. Current research is looking into the impacts of a richer social environment for these twins to stimulate their development of language.^[94]

Animals[edit]

Twins are common in many mammal species, including cats, sheep, ferrets, giant pandas, cows, dolphins, dogs, deer, marmosets, tamarins, and elephants. The incidence of twinning among cattle is about 1–4%, and research is underway to improve the odds of twinning, which can be more profitable for the breeder if complications can be sidestepped or managed. A female calf that is the twin of a bull becomes partially masculinized and is known as a freemartin.

See also[edit]

References[edit]

Further reading[edit]

• Bacon, Kate. Twins in Society: Parents, Bodies, Space, and Talk (Palgrave Macmillan; 2010) 221 pages; explores the experience of child twins, adult twins, and parents of twins, with a focus on Britain.
• Nieuwint, Aggie; Van Zalen-Sprock, Rieteke; Hummel, Pieter; Pals, Gerard; Van Vugt, John; et al. (1999). «‘Identical’ twins with discordant karyotypes». Prenatal Diagnosis. 19 (1): 72–6. doi:10.1002/(SICI)1097-0223(199901)19:1<72::AID-PD465>3.0.CO;2-V. PMID 10073913. S2CID 19112883.
• Wenk RE, Houtz T, Brooks M, Chiafari FA (1992). «How frequent is heteropaternal superfecundation?». Acta Geneticae Medicae et Gemellologiae. 41 (1): 43–7. doi:10.1017/S000156600000249X. PMID 1488855. S2CID 23507167.
• Girela, Eloy; Lorente, Jose A.; Alvarez, J. Carlos; Rodrigo, Maria D.; Lorente, Miguel; et al. (1997). «Indisputable double paternity in dizygous twins». Fertility and Sterility. 67 (6): 1159–61. CiteSeerX 10.1.1.378.4082. doi:10.1016/S0015-0282(97)81456-2. PMID 9176461.
• Shinwell ES, Reichman B, Lerner-Geva L, Boyko V, Blickstein I (September 2007). ««Masculinizing» effect on respiratory morbidity in girls from unlike-sex preterm twins: a possible transchorionic paracrine effect». Pediatrics. 120 (3): e447–53. doi:10.1542/peds.2006-3574. PMID 17766488. S2CID 20498737. Retrieved 2008-10-06.
• Lummaa V, Pettay JE, Russell AF (June 2007). «Male twins reduce fitness of female co-twins in humans». Proceedings of the National Academy of Sciences of the United States of America. 104 (26): 10915–20. Bibcode:2007PNAS..10410915L. doi:10.1073/pnas.0605875104. PMC 1904168. PMID 17576931.
• Schein, Elyse; Paula Bernstein (2007). Identical Strangers: A Memoir of Twins Separated and Reunited. New York: Random House. ISBN 978-1-4000-6496-0. OCLC 123390922.
• Helle, Samuli; Virpi Lummaa; Jukka Jokela (2004). «Selection for Increased Brood Size in Historical Human Populations» (PDF). Evolution. 58 (2): 430–436. doi:10.1111/j.0014-3820.2004.tb01658.x. PMID 15068359. S2CID 9460009. Archived from the original (PDF) on 2008-10-02. Retrieved 2008-10-02.
• «TWINS Guide to the First Year» (PDF). TWINS Magazine. Fort Collins, Colorado. 2008. Archived from the original (PDF) on 2008-10-30. Retrieved 2008-10-06.
• Samson, Jennifer. «Facts About Multiples: An Encyclopedia of Multiple Birth Records». Archived from the original on 2009-10-15. Retrieved 2008-10-18.
• Taylor, Ally. «Twin Zygosity Test for Dichorionic Diamniotic (Di/Di) Twins! Zygosity Reveal». Retrieved 2018-03-30.
• Am J Med Genet C Semin Med Genet. 2009 May 15;151C(2):136-41. Not really identical: epigenetic differences in monozygotic twins and implications for twin studies in psychiatry. Haque FN, Gottesman II, Wong AH.
• Twin brothers promoted as Majors General together. Seneviratne brothers, who are twins and joined the Army on a same day, were promoted to the rank of Major General, again on the same day.
• Guinness World Record holding twins: https://www.guinnessworldrecords.com/news/2022/11/theyre-so-different-even-though-theyre-identical-twins-725135

External links[edit]

• Twins — news and articles

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I was born in New York City, along with a twin sister. I am five minutes older than Emily. It was Emily, for reasons no one knows — she certainly doesn’t — who called me Avi. It stuck. It’s the only name I use now.

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ETYMOLOGY OF THE WORD TWIN

Old English twinn; related to Old High German zwiniling twin, Old Norse tvinnr double.

PRONUNCIATION OF TWIN

GRAMMATICAL CATEGORY OF TWIN

Twin is a verb and can also act as a noun.

WHAT DOES TWIN MEAN IN ENGLISH?

Definition of twin in the English dictionary

The first definition of twin in the dictionary is either of two persons or animals conceived at the same time. Other definition of twin is either of two persons or things that are identical or very similar; counterpart. Twin is also Also called: macle. a crystal consisting of two parts each of which has a definite orientation to the other.

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SYNONYMS OF «TWIN»

The following words have a similar or identical meaning as «twin» and belong to the same grammatical category.

TRANSLATION OF TWIN

Find out the translation of twin to 25 languages with our English multilingual translator.

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The term «twin» is very widely used and occupies the 3.666 position in our list of most widely used terms in the English dictionary.

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10 ENGLISH BOOKS RELATING TO «TWIN»

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10 NEWS ITEMS WHICH INCLUDE THE TERM «TWIN»

Find out what the national and international press are talking about and how the term twin is used in the context of the following news items.

REFERENCE

« EDUCALINGO. Twin [online]. Available <https://educalingo.com/en/dic-en/twin>. Apr 2023 ».

If one twin is autistic, it’s twelve times more likely than the average rate that the other twin is autistic. ❋ Unknown (2010)

In fact, not only does the US government run an internal account deficit (1), or CAD, but the twin is at 700 billion USD a year! ❋ Unknown (2005)

LIN: For our viewers who are just tuning in right now, you are looking at live picture of the World Trade Center tower, where, according to eyewitness Sean Murtagh — he is the vice president of finance and eyewitness to what he describes as a twin-engine plane — or possibly a 737 passenger jet — flying into the World Trade Center. ❋ Unknown (2001)

LIN: Jeanne, we are continuing to look at pictures of this devastating scene, according to Sean Murtagh, vice president of finance, who witnessed what he described as a twin-engine plane, possibly a 737. e was almost absolutely sure it was a large passenger jet that went into that. ❋ Unknown (2001)

We have endorsed what they call the twin-track proposal by the two governments in Northern Ireland, which is basically a proposal to have prepatory talks of varying levels move forward at the same time that there would be an international body to address the issue of arms that has become an obstacle to moving towards all party talks. ❋ ITY National Archives (1995)

Jackson said the deal was perhaps the last, best opportunity to address «critical issues» in global communications, which he called the twin national challenges of «creating jobs and helping to connect every American, especially people of color, to vitally needed news, information and broadband internet services.» ❋ Unknown (2010)

In Los Angeles, developer Sonny Astani was well on his way to completing the first phase of his Concerto high-rise project, planned to include 629 residential units in twin 30-story towers. ❋ Halsey Minor (2010)

The other twin is larger and has too much amniotic fluid around it and is called the «recipient» twin. ❋ Unknown (2010)

There has been tremendous advancement in twin cams models but the single cam is still the easiest bow to keep shooting tight groups long term. ❋ Unknown (2010)

Nigerian authorities have arrested the brother of former militant leader Henry Okah, on suspicion that was involved in twin car bombings that killed 12 people. ❋ Unknown (2010)

One twin is smaller with little amniotic fluid around it and is called the «donor» twin. ❋ Unknown (2010)

Parasitic twins generally do not improve the life of the main twin, not just because it is uncomfortable or embarrassing, but because it is a drain on their health and resources. ❋ Heather McDougal (2009)

And, like you have written here, with women the twin is in their uterus or ovaries and with men somewhere in their intestines. ❋ Heather McDougal (2009)

My ugly twin is laughing that people who have not filed tax for years (avoiding repayment of HECS) will be spewy and resentful ❋ Another Outspoken Female (2009)

— “Something of the Greenglow”: usually, the energy-twin is a glowing green specter of the person, but I suppose it could also be the blob of energy shown here. ❋ Unknown (2009)

[What up] twin, [you down] for [tonight]? ❋ Joker75 (2006)

Person One: Wow anna seems so awesome and unique.
Person two:Yeah,sucks that she’s [a twin] and she can never talk to people without the words «[Twin]» in it.
Person One: [Wow sucks] for her. ❋ The Only Sane One (2007)

[Rima]: This [party’s] whack
[Samar]: Yeah, let’s twin. ❋ Samar And Rima (2008)

if u [pitch in] 5 extra [dollars], we can get [a twin] ❋ Melvin_flynt_da Hustla (2003)

You [got] [a twin]? ❋ Dakota Bongwater (2004)

[hey] [mark] [you are a] twin ❋ Mudafucka (2005)

[what up] twin ❋ YoungCali (2003)

[Joe]: How much money do you have?
[John Paul]: All I got is [a twin]. ❋ Tw-in (2003)

[Chelsea]: I saw [Landon] again today.
Brandon: Really, because I just saw Prelsea a little bit ago.
Chelsea: If we weren’t [twin] twins we wouldn’t see each others twin so often. ❋ Bravycrockett (2010)

DEO goat: [luh you] twin[Yrg] goat: thanks [gangy] ❋ DEOZ (2022)