Twin

Monozygotic twins

A twin is one of two offspring produced in the same pregnancy.[1]

Twins can either be monozygotic (MZ, colloquially "identical"), meaning that they develop from one zygote that splits and forms two embryos, or dizygotic (DZ, colloquially "fraternal") because they develop from two separate eggs that are fertilized by two separate sperm.

In contrast, a fetus which develops alone in the womb is called a singleton, and the general term for one offspring of a multiple birth is multiple.

Contents

Statistics

Twins are estimated to be approximately 1.9% of the world population, with monozygotic twins making up 0.2% of the total - and 8% of all twins.

The twin birth rate in the United States is slightly above 32 twin live births per 1,000 live births,[2] while the Yoruba have the highest rate of twinning in the world, at 45 twins per 1,000 live births,[3][4][5] 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.[6][7]

Due to the limited size of the mother's womb, multiple pregnancies are much less likely to carry to full term than single births, with twin pregnancies lasting only 37 weeks (3 weeks less than full term) on average.[8]

Zygosity

Zygosity is the degree of identity in the genome of twins. There are five common variations of twinning. The three most common variations are all dizygotic:

The other two variations are monozygotic 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,[9] while it is 1.07 males/female in Italy.[10] However, males are also more susceptible than females to death in utero, and since the death rate in utero is higher for twins, it leads to female twins being more common than male twins.

Eight month old dizygotic twin girls napping

Dizygotic twins

Dizygotic twins (commonly known as fraternal twins, but also referred to as non-identical twins or biovular twins) usually occur when two fertilized eggs are implanted in the uterine wall at the same time. When two eggs are independently fertilized by two different sperm cells, DZ twins result. The two eggs, or ova, form two zygotes, hence the terms dizygotic and biovular.

Dizygotic twins, like any other siblings, have an extremely small chance of having the same chromosome profile. Like any other siblings, DZ twins may look similar, particularly given that they are the same age. However, DZ twins may also look very different from each other. They may be of different sexes or the same sex. The same holds true for brothers and sisters from the same parents, meaning that DZ twins are simply brothers and/or sisters who happen to be the same age.

Studies show that there is a genetic basis for DZ twinning. However, it is only their mother that has any effect on the chances of having DZ 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.[6]

DZ twins are also more common for older mothers, with twinning rates doubling in mothers over the age of 35.[11] With the advent of technologies and techniques to assist women in getting pregnant, the rate of fraternals has increased markedly. For example, in New York City's Upper East Side there were 3,707 twin births in 1995; there were 4,153 in 2003; and there were 4,655 in 2004. Triplet births have also risen, from 60 in 1995 to 299 in 2004.

Monozygotic twins

Comparison of zygote development in monozygotic and dizygotic twins. In the uterus, a majority of monozygotic twins (60–70%) share the same placenta but have separate amniotic sacs. In 18–30% of monozygotic twins each fetus has a separate placenta and a separate amniotic sac. A small number (1–2%) of monozygotic twins share the same placenta and amniotic sac. Dizygotic twins each have their own placenta and own amniotic sac.

Monozygotic twins, frequently referred to as identical twins, occur when a single egg is fertilized to form one zygote (monozygotic) which then divides into two separate embryos.

There are an estimated 10 million identical twins and triplets in the world today.

Mechanism

Regarding spontaneous or natural monozygotic twinning, a recent theory posits that monozygotic twins are formed after a blastocyst essentially collapses, splitting the progenitor cells (those that contain the body's fundamental genetic material) in half, leaving the same genetic material divided in two on opposite sides of the embryo. Eventually, two separate fetuses develop.[12] Spontaneous division of the zygote into two embryos is not considered to be a hereditary trait, but rather a spontaneous or random event.[11][13]

Monozygotic twins may also be created artificially by embryo splitting. It can be used as an expansion of IVF to increase the number of available embryos for embryo transfer.[14]

Incidence

Monozygotic twinning occurs in birthing at a rate of about three in every 1000 deliveries worldwide,[13]

The likelihood of a single fertilization resulting in MZ twins is uniformly distributed in all populations around the world.[11] This is in marked contrast to DZ twinning, which ranges from about six per thousand births in Japan (almost similar to the rate of MZ twins, which is around 4–5) to 15 and more per thousand in some parts of India[15] and up to 24 in the US, which might mainly be due to IVF (in vitro fertilisation). The exact cause for the splitting of a zygote or embryo is unknown.

In-vitro fertilization techniques are more likely to create twins. Only about three pairs of twins per 1,000 deliveries occur as a result of natural conception, while for IVF deliveries, there are nearly 21 pairs of twins for every 1,000.[16]

Genetic and epigenetic similarity

Monozygotic twins are genetically identical (unless there has been a mutation during development) and they are always the same sex. On rare occasions, monozygotic twins may express different phenotypes (normally due to an environmental factor or the deactivation of different X chromosomes in monozygotic female twins), and in some extremely rare cases, due to aneuploidy, twins may express different sexual phenotypes, normally due to an XXY Klinefelter's syndrome zygote splitting unevenly.[17][18]

Monozygotic twins have nearly identical DNA, but differing environmental influences throughout their lives affect which genes are switched on or off. This is called epigenetic modification. A study of 80 pairs of human twins ranging in age from three to 74 showed that the youngest twins have relatively few epigenetic differences. The number of epigenetic differences between MZ twins 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.[19] However, certain characteristics become more alike as twins age, such as IQ and personality.[20][21] This phenomenon illustrates the influence of genetics in many aspects of human characteristics and behaviour.

Phenotype similarity

Monozygotic twins are almost always the same sex and their traits and physical appearances are very similar but not exactly the same.[22]

Monozygotic twins look alike, although they do not have the same fingerprints (which are environmental as well as genetic). As they mature, MZ twins often become less alike because of lifestyle choices or external influences. The children of MZ twins would test genetically as half-siblings rather than first cousins.

Half twin

Main article: Half twin

The extremely rare half twins or semi-identical twins are twins that inherit exactly the same genes from their mother but different genes from their father. The exact mechanism of their conception is not well understood, but could theoretically occur in polar body twinning where sperm cells fertilize both the ovum and the second polar body.

Degree of separation

Various types of chorionicity and amniosity (how the baby's sac looks) in monozygotic (one egg/identical) twins as a result of when the fertilized egg divides

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 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. 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 means twins are dizygotic (non-identical). But if monozygotic twins separate early enough, the arrangement of sacs and placentas in utero is indistinguishable from 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[23] ), in 99.7% of all pregnancies,[24] and in 18–36%[25] (or around 25%[23]) 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.[26]

 Dichorionic-Diamniotic twins form when splitting takes place by the third day after fertilization.[23]
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.[25] Monochorionic-Diamniotic twins are almost always monozygotic, with a few exceptions where the blastocysts have fused.[23]

 Days 4-8
Monochorionic-Monoamniotic Sometimes, monochorionic twins also share the same amniotic. This situation occurs in 1–2% of monozygotic twin pregnancies.[25]

Monoamniotic twins are always monozygotic (identical twins).[27]

The survival rate for monoamniotic twins is somewhere between 50%[27] to 60%.[28]

Consequently, if twins are monoamniotic that means that the two babies will be sharing a placenta and as a result, due to the small capacity of sharing a sac, the umbilical cord has 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 the lack of oxygen.

 Monoamniotic twins occur when the split takes place after the ninth day after fertilization.[23]
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.

Demographics

A recent 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."[29]

From 1980–97, the number of twin births in the United States rose 52%.[30] This rise can at least partly be attributed to the increasing popularity of fertility drugs like Clomid and procedures such as in vitro fertilization, which result in multiple births more frequently than unassisted fertilizations do. It may also be linked to the increase of growth hormones in food.[29]

Ethnicity

About 1 in 90 human births (1.1%) results from a twin pregnancy.[31] The rate of dizygotic twinning varies greatly among ethnic groups, ranging as high as about 45 per 1000 births for the Yoruba to 10% for Linha São Pedro, a tiny Brazilian settlement which belongs to the city of Cândido Godói.[32] In Cândido Godói, one in five pregnancies have resulted in twins.[33] The Argentine historian Jorge Camarasa has put forward a theory that experiments of the Nazi doctor Josef Mengele could be responsible for the high ratio of twins in the area.[34] 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.[35] A high twinning rate has also been observed in other places of the world, including Igbo-Ora in Nigeria[36] and Kodinji in India.[37]

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 per cent of twins were monozygous. 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.[38]

Predisposing factors

The predisposing factors of monozygotic twinning are unknown.

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

Women undergoing certain fertility treatments may have a greater chance of dizygotic multiple births. This 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. Some other treatments such as the drug Clomid can stimulate a woman to release multiple eggs, increasing the likelihood of multiples.

Delivery interval

A 15-year German study[39] of 8,220 vaginally delivered twins (that is, 4,110 pregnancies) in Hesse yielded a mean delivery time interval of 13.5 minutes.[40] 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% (72 instances)

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.

Complications of twin pregnancy

Vanishing twins

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 has died very early in the pregnancy and has not been detected or recorded.[41] Early obstetric ultrasonography exams sometimes reveal an "extra" fetus, which fails to develop and instead disintegrates and vanishes. This is known as vanishing twin syndrome.

Chang and Eng Bunker, born in Siam (now Thailand) in 1811, were the origin of the term "Siamese twins".

Conjoined twins

Main article: Conjoined twin

Conjoined twins (or the deprecated term "Siamese twins") are monozygotic twins whose bodies are joined together during pregnancy. This occurs where the single zygote of MZ twins fails to separate completely, and the zygote starts to split after day 12[23] following fertilization. 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 or liver.

Chimerism

Main article: Chimera (genetics)

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 addition, in certain cases the person or chimera may have two sets of DNA.

Parasitic twins

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 medically dealt with.

Partial molar twins

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

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 than the vanishing twin syndrome.

Low birth weight

Twins typically suffer from the lower birth weights and greater likelihood of prematurity that is more commonly associated with the higher multiple pregnancies. Throughout their lives twins tend to be smaller than singletons on average.

Twin-to-twin transfusion syndrome

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.

Human twin studies

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. However, the utility and accuracy of these twin studies has been called into question and remains controversial. Classical twin studies have are now being supplemented with molecular genetic studies which identify individual genes.

Unusual twinnings

Among dizygotic twins, in rare cases, the eggs are fertilized at different times with two or more acts of sexual intercourse, either within one menstrual cycle (superfecundation) or, even more rarely, later on in the pregnancy (superfetation). This can lead to the possibility of a woman carrying fraternal twins with different fathers (that is, half-siblings). 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%; see the references section, below, for more details.

Dizygotic twins from biracial couples can sometimes be mixed twins, which exhibit differing ethnic and racial features. One such pairing was born in Germany in 2008 to a white father from Germany and a black mother from Ghana.[42]

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.

Among monozygotic twins, in extremely rare cases, twins have been born with opposite sexes (one male, one female). The probability of this is so vanishingly small (only 3 documented cases[43]) that multiples having different sexes is universally accepted as a sound basis for a clinical determination that in utero multiples are not monozygotic. When monozygotic twins are born with different sexes it is because of chromosomal birth defects. In this case, although the twins did come from the same egg, it is incorrect to refer to them as genetically identical, since they have different karyotypes.

Semi-identical twins

See also: half twin

Monozygotic twins can develop differently, due to different genes being activated.[44] More unusual are "semi-identical twins". These "half-identical twins" are hypothesized to occur when an unfertilized egg cleaves into two identical attached ova and which are viable for fertilization. Both cloned ova are then fertilized by different sperm and the coalesced eggs undergo further cell duplications developing as a chimeric blastomere. If this blastomere then undergoes a twinning event, two embryos will be formed, each of which have different paternal genes and identical maternal genes.

This results in a set of twins with identical genes from the mother's side, but different genes from the father's side. Cells in each fetus carry genes from either sperm, resulting in chimeras. This form had been speculated until only recently being recorded in western medicine.[45][46][47]

Twin calves of the Hereford breed

Animal twins

Twins are common in many animal species, such as cats, sheep, ferrets and deer. The incidence of twinning among cattle is about 1–4%, and research is under way to improve the odds of twinning, which can be more profitable for the breeder if complications can be sidestepped or managed. The nine-banded armadillo (Dasypus novemcinctus) has identical twins (usually four babies) as its regular reproduction and not as exceptional cases.[48][49]

Mythology

See also

References

  1. ↑ MedicineNet > Definition of Twin Last Editorial Review: 6/19/2000
  2. ↑ Martin, Joyce A.; Hamilton, Brady E.; Sutton, Paul D.; Ventura, Stephanie J.; Menacker, Fay; Kirmeyer, Sharon and Mathews, T.J. Births: Final Data for 2006, National Vital Statistics Report, Vol. 57, No. 7, Jan. 7, 2009, 102 pp. (Table 39, pp.83-84)
  3. ↑ Zach, Terence; Arun K Pramanik and Susannah P Ford (2007-10-02). "Multiple Births". WebMD. http://www.emedicine.com/PED/topic2599.htm. Retrieved 2008-09-29. 
  4. ↑ "Genetics or yams in the Land of Twins?". Independent Online. 2007-11-12. http://www.iol.co.za/index.php?from=rss_Africa&set_id=1&click_id=68&art_id=nw20071112091221822C182549. Retrieved 2008-09-29. 
  5. ↑ "The Land Of Twins". BBC World Service. 2001-06-07. http://www.bbc.co.uk/worldservice/people/highlights/010607_twins.shtml. Retrieved 2008-09-29. 
  6. ↑ 6.0 6.1 O. Bomsel-Helmreich; W. Al Mufti (1995). "The mechanism of monozygosity and double ovulation". In Louis G. Keith, Emile Papierik, Donald M. Keith and Barbara Luke. Multiple Pregnancy: Epidemiology, Gestation & Perinatal Outcome. Taylor and Francis. pp. 34. ISBN 1-85070-666-2. 
  7. ↑ "What’s in a yam? Clues to fertility, a student discovers". 1999. http://yalemedicine.yale.edu/ym_su99/scope/scope1.htm#yam. Retrieved 2008-09-29. 
  8. ↑ Elliott, JP (December 2007). "Preterm labor in twins and high-order multiples.". Clinical Perinatology 34 (4): 599–609. doi:10.1016/j.clp.2007.10.004. PMID 18063108. "Unlike singleton gestation where identification of patients at risk for PTL is often difficult, every multiple gestation is at risk for PTL, so all patients can be managed as being at risk.". 
  9. ↑ "United States: People". The World Factbook. Central Intelligence Agency. 2008-09-04. https://www.cia.gov/library/publications/the-world-factbook/geos/us.html#People. Retrieved 2008-10-02. 
  10. ↑ "Italy: People". The World Factbook. Central Intelligence Agency. 2008-09-04. https://www.cia.gov/library/publications/the-world-factbook/geos/it.html#People. Retrieved 2008-10-02. 
  11. ↑ 11.0 11.1 11.2 Bortolus, Renata; Fabio Parazzini, Liliane Chatenoud, Guido Benzi, Massimiliano Maria Bianchi, Alberto Marini (1999). "The epidemiology of multiple births". Human Reproduction Update (European Society of Human Reproduction and Embryology) 5 (2): 179–187. doi:10.1093/humupd/5.2.179. ISSN 1355-4786. PMID 10336022. 
  12. ↑ Cheng, Maria; Associated Press (2007-07-03). "Study: Twins form after embryo collapses". USA Today. http://www.usatoday.com/news/health/2007-07-03-52152171_x.htm. Retrieved 2008-09-30. 
  13. ↑ 13.0 13.1 April Holladay (2001-05-09). "What triggers twinning?". WonderQuest. http://www.wonderquest.com/TwinsTrigger.htm. Retrieved 2007-03-22. 
  14. ↑ Illmensee K, Levanduski M, Vidali A, Husami N, Goudas VT (February 2009). "Human embryo twinning with applications in reproductive medicine". Fertil. Steril. 93 (2): 423–7. doi:10.1016/j.fertnstert.2008.12.098. PMID 19217091. 
  15. ↑ Oleszczuk, Jaroslaw J.; Donald M. Keith, Louis G. Keith and William F. Rayburn (November 1999). "Projections of population-based twinning rates through the year 2100". The Journal of Reproductive Medicine 44 (11): 913–921. PMID 10589400. http://www.reproductivemedicine.com/toc/auto_abstract.php?id=13594. Retrieved 2008-10-02. 
  16. ↑ European Society of Human Reproduction and Embryology (2007-07-02). "Time-lapse recordings reveal why IVF embryos are more likely to develop into twins. Researchers believe the laboratory culture could be the cause". Press release. http://www.eshre.com/emc.asp?pageId=939. Retrieved 2008-09-30. 
  17. ↑ Edwards, JH; T Dent and J Kahn (June 1966). "Monozygotic twins of different sex". Journal of Medical Genetics 3 (2): 117–123. doi:10.1136/jmg.3.2.117. PMID 6007033. 
  18. ↑ Machin, GA (January 1996). "Some causes of genotypic and phenotypic discordance in monozygotic twin pairs". American Journal of Medical Genetics 61 (3): 216–228. doi:10.1002/(SICI)1096-8628(19960122)61:3<216::AID-AJMG5>3.0.CO;2-S. PMID 8741866. 
  19. ↑ Fraga, Mario F.; Esteban Ballestar, Maria F. Paz, Santiago Ropero, Fernando Setien, Maria L. Ballestar†, Damia Heine-Suñer, Juan C. Cigudosa, Miguel Urioste, Javier Benitez, Manuel Boix-Chornet, Abel Sanchez-Aguilera, Charlotte Ling, Emma Carlsson, Pernille Poulsen, Allan Vaag, Zarko Stephan, Tim D. Spector, Yue-Zhong Wu, Christoph Plass and Manel Esteller (July 2005). "Epigenetic differences arise during the lifetime of monozygotic twins". Proc. Natl. Acad. Sci. U.S.A. 102 (30): 10604–9. doi:10.1073/pnas.0500398102. PMID 16009939. PMC 1174919. http://www.pnas.org/content/102/30/10604.full. Retrieved 2008-09-29. 
  20. ↑ Segal, Nancy L. (1999). Entwined lives: twins and what they tell us about human behavior. New York: Dutton. ISBN 0-525-94465-6. OCLC 40396458. 
  21. ↑ Plomin, Robert (2001). Behavioral genetics. New York: Worth Pubs. ISBN 0-7167-5159-3. OCLC 43894450. 
  22. ↑ LiveScience (2008-02-21). "'Identical' twins? Not according to their DNA". MSNBC. http://www.msnbc.msn.com/id/23276953/. Retrieved 2008-10-10. 
  23. ↑ 23.0 23.1 23.2 23.3 23.4 23.5 Shulman, Lee S.; Vugt, John M. G. van (2006). Prenatal medicine. Washington, DC: Taylor & Francis. pp. Page 447. ISBN 0-8247-2844-0. 
  24. ↑ Cordero L, Franco A, Joy SD, O'shaughnessy RW (December 2005). "Monochorionic diamniotic infants without twin-to-twin transfusion syndrome". J Perinatol 25 (12): 753–8. doi:10.1038/sj.jp.7211405. PMID 16281049. 
  25. ↑ 25.0 25.1 25.2 Curran, Mark (2005-11-02). "Twinning". Focus Information Technology. http://www.obfocus.com/high-risk/twins/twin1.1.htm. Retrieved 2008-10-10. 
  26. ↑ Benirschke, Kurt (2004). "Multiple Gestation". In Robert Resnik, Robert K. Creasy and Jay D. Iams. Maternal-Fetal Medicine: Principles and Practice (5th ed.). Philadelphia: W. B. Saunders Company. pp. 55–62. ISBN 0-7216-0004-2. 
  27. ↑ 27.0 27.1 Pregnancy-Info -- > Monoamniotic Twins Retrieved on July 9, 2009
  28. ↑ MoMo Twins; Monochorionic Monoamniotic Twins By Pamela Prindle Fierro, About.com. Retrieved on July 9, 2009
  29. ↑ 29.0 29.1 Steinman, Gary (May 2006). "Mechanisms of twinning: VII. Effect of diet and heredity on the human twinning rate". J Reprod Med 51 (5): 405–410. PMID 16779988. http://www.reproductivemedicine.com/toc/auto_abstract.php?id=22626. Retrieved 2008-09-29. 
  30. ↑ Martin, Joyce A; Melissa M. Park (1999-09-14). "Trends in Twin and Triplet Births: 1980–97" (PDF). National Vital Statistics Reports (National Center for Health Statistics) 47 (24): 1–17. http://www.cdc.gov/nchs/data/nvsr/nvsr47/nvs47_24.pdf. Retrieved 2008-09-29. 
  31. ↑ Asch, Richard H.; John Studd (1995). Progress in Reproductive Medicine Volume II. Informa. ISBN 1-85070-574-7. OCLC 36287045. 
  32. ↑ Matte, U; MG Le Roux, B Bénichou, JP Moisan and R Giugliani (1996). "Study on possible increase in twinning rate at a small village in south Brazil". Acta Genet Med Gemellol (Roma) 45 (4): 431–437. PMID 9181177. 
  33. ↑ Nick Evans (21 January 2009big). "Nazi Angel of death Josef Mengel created twin town in Brazil". The Telegraph. http://www.telegraph.co.uk/news/worldnews/southamerica/brazil/4307262/Nazi-angel-of-death-Josef-Mengele-created-twin-town-in-Brazil.html. 
  34. ↑ Caroline Iggulden (23 January 2009). "The twins from Brazil". The Sun. http://www.thesun.co.uk/sol/homepage/woman/real_life/article2162918.ece. 
  35. ↑ Linda Geddes: Nazi 'Angel of Death' not responsible for town of twins New Scientist online, 27 January 2009
  36. ↑ "The Land Of Twins". BBC World News. 7 June 2001. http://www.bbc.co.uk/worldservice/people/highlights/010607_twins.shtml. Retrieved 2009-08-06. 
  37. ↑ "Seeing double: the village in deepest Kerala where twins have taken over". The Independent. 12 May 2009. http://www.independent.co.uk/news/science/seeing-double-the-village-in-deepest-kerala-where-twins-have-taken-over-1683129.html. Retrieved 2009-08-06. 
  38. ↑ Rehan, N; DS Tafida (November 1980). "Multiple births in Hausa women". Br J Obstet Gynaecol 87 (11): 997–1004. doi:10.1111/j.1471-0528.1980.tb04464.x. PMID 7437372. 
  39. ↑ "Twin-to-twin delivery time interval: influencing factors and effect on short-term outcome of the second twin"; Werner Stein, Björn Misselwitz & Stefan Schmidt; Acta Obstetricia et Gynecologica Scandinavica, 87(3):346-353, 2008.
  40. ↑ There were 836,104 deliveries of babies in Hesse over the 15-year study period, including 11,740 twin pregnancies, of which only 4,110 met the inclusion criteria and hence were examined in the study. The excluded twin pregnancies were in cases of (1) delivery before 34 weeks of gestation; (2) when the first twin was delivered by caesarean section; (3) when either of the twins had died in the womb before the onset of labor; and (4) when the pregnancy had been complicated by fetal malformations or Twin-to-twin transfusion syndrome.
  41. ↑ Gedda, Luigi (1995). "The role of research in twin medicine". In Louis G. Keith, Emile Papiernik, Donald M. Keith and Barbara Luke. Multiple Pregnancy: Epidemiology, Gestation & Perinatal Outcome. Taylor and Francis. pp. 4. ISBN 1-85070-666-2. 
  42. ↑ Lovell, Tammy (2008-07-17). "Pictured: Proud parents show off their million-to-one black and white twins". Daily Mail (Associated Newspapers Ltd). http://www.dailymail.co.uk/femail/article-1035939/Pictured-Proud-parents-million-black-white-twins.html. Retrieved 2008-10-02. 
  43. ↑ Schmidt, R; EH Sobel, HM Nitowsky, H Dar and FH Allen Jr (February 1976). "Monozygotic twins discordant for sex". Journal of Medical Genetics 13 (1): 64–68. doi:10.1136/jmg.13.1.64. PMID 944787. PMC 1013354. http://jmg.bmj.com/cgi/content/abstract/13/1/64. Retrieved 2008-09-29. 
  44. ↑ Gilbert, Scott F. (2006). "Non-identical Monozygotic Twins". Developmental biology. Sunderland, Mass: Sinauer Associates. ISBN 0-87893-250-X. OCLC 172964621. http://8e.devbio.com/article.php?id=111. Retrieved 2008-09-30. 
  45. ↑ Souter, Vivienne L.; Melissa A. Parisi, Dale R. Nyholt, Raj P. Kapur, Anjali K. Henders, Kent E. Opheim, Daniel F. Gunther, Michael E. Mitchell, Ian A. Glass and Grant W. Montgomery (April 2007). "A case of true hermaphroditism reveals an unusual mechanism of twinning". Hum. Genet. 121 (2): 179–85. doi:10.1007/s00439-006-0279-x. PMID 17165045. 
  46. ↑ LiveScience Staff (2007-03-26). "Rare Semi-Identical Twins Discovered". Imaginova. http://www.livescience.com/health/070326_semi_twins.html. Retrieved 2008-10-01. 
  47. ↑ Whitfield, John (2007). "'Semi-identical' twins discovered". Nature. doi:10.1038/news070326-1. 
  48. ↑ Cyranoski D. (2009). Developmental biology: Two by two. Nature. 458(7240):826-9.PubMed
  49. ↑ Newman HH. Patterson JT. (1909). A Case of Normal Identical Quadruplets in the Nine-Banded Armadillo, and Its Bearing on the Problems of Identical Twins and of Sex Determination. Biological Bulletin, 17: 181-187 JSTOR 1536114

Further reading

Twin conditions (Q89.3-Q89.4, 759.3-759.4)
Zygosity
Twin  · Half twin
Monochorionic twins
Twin-to-twin transfusion syndrome  · Twin reversed arterial perfusion  · Monoamniotic twins
Conjoined twins
Parasitic twin (Craniopagus parasiticus, Fetus in fetu)
Other
Vanishing twin  · Chimerism