A twin is one of two offspring produced in the same pregnancy. Twins can either be monozygotic ("identical"), meaning that they develop from one zygote that splits and forms two embryos, or dizygotic ("fraternal"), meaning that they develop from two eggs, each fertilized by separate sperm cells.
- 1 Statistics
- 2 Zygosity
- 3 Degree of separation
- 4 Demographics
- 5 Delivery interval
- 6 Complications of twin pregnancy
- 7 Management of twin pregnancies
- 8 Human twin studies
- 9 Language development in twins
- 10 Animal twins
- 11 See also
- 12 References
- 13 Further reading
The twin birth rate in the United States rose 76% from 1980 through 2009, from 18.9 to 33.3 per 1,000 births. The Yoruba have the highest rate of twinning in the world, at 45-50 twin sets (or 90-100 twins) per 1,000 live births, 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.
The world's highest rates of twinning are found across Central Africa, where there are 18-30 twin sets (or 36-60 twins) per 1,000 live births. In Latin-America, South Asia and South-East Asia the lowest rates are found; only 6-9 twin sets per 1,000 live births. North America and Europe have intermediate rates of 9–16 twin sets per 1,000 live births.
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.
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. Other factors that increase the odds of having fraternal twins include maternal age, fertility drugs and other fertility treatments, nutrition, and prior births.
The three most common variations are all dizygotic (fraternal):
- Male–female twins are the most common result, 50 percent of dizygotic twins and the most common grouping of twins.
- Female–female dizygotic twins (sometimes called "sororal twins")
- Male–male dizygotic twins
The other two variations are monozygotic ("identical") twins:
- Female–female monozygotic twins
- Male–male monozygotic twins (less common)
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, while it is 1.07 males/female in Italy. 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.
Fraternal (sororal; dizygotic) twins
Fraternal or dizygotic (DZ) twins (also referred to as "non-identical twins", "dissimilar twins", "biovular twins", and, in cases 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.
Dizygotic twins, like any other siblings, have an extremely small chance of having the same chromosome profile. Like any other siblings, dizygotic twins may look similar, particularly given that they are the same age. However, dizygotic 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 fraternal and/or sororal twins are simply siblings who happen to be the same age.
Studies show that there is a genetic basis 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.
Dizygotic twins are also more common for older mothers, with twinning rates doubling in mothers over the age of 35. With the advent of technologies and techniques to assist women in getting pregnant, the rate of fraternals has increased markedly.
Monozygotic ("identical") twins
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. Spontaneous division of the zygote into two embryos is not considered to be a hereditary trait, but rather a spontaneous or random event.
Monozygotic twinning occurs in birthing at a rate of about three in every 1000 deliveries worldwide.
The likelihood of a single fertilization resulting in monozygotic twins is uniformly distributed in all populations around the world. 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 and up to over 20 in some Central African countries. The exact cause for the splitting of a zygote or embryo is unknown.
Genetic and epigenetic similarity
Monozygotic twins are genetically nearly identical and they are always the same 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, due to the fact that even in a small space inside the womb, people have contact with different parts of this environment, which gives small variations in the same digital, making them unique. Monozygotic twins always have different phenotypes. 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.
Although monozygotic twins are genetically very similar, a study of 92 pairs of monozygotic twins, carried out in November 2012, has found that monozygotic twins acquire hundreds of genetic differences early in fetal development, due to mutations (or copy errors) taking place in the DNA of each twin after the splitting of the embryo. It is estimated that, on average, a set of monozygotic twins will have about 360 genetic differences that occurred early in fetal development.
Another cause of difference between monozygotic twins is epigenetic modification, caused by differing environmental influences throughout their lives, which affects which genes are switched on or off. 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. However, certain characteristics become more alike as twins age, such as IQ and personality. This phenomenon illustrates the influence of genetics in many aspects of human characteristics and behavior.
Half-identical or semi-identical twins (also referred to as "half twins") are the result of a very rare form of twinning in which the twins inherit exactly the same genes from their mother but different genes from their father. Although examples of half-identical twins have been found, 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.
This situation is not the same as the common form of fraternal twinning, in which two genetically different ova are fertilized by two genetically different sperm. In this case, the ova are genetically identical.
There are two mechanisms by which this might happen:
- Polar twins (or "polar body twins"), where two sperm fertilize an ovum, one of the two fertilizing a polar body; or where an ovum splits into identical copies, one containing a polar body, prior to fertilization, allowing it to be fertilized by two different sperm.
- Sesquizygotic twins, where two sperm fertilize the one ovum, forming a triploid, and then splitting.
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 likely a polar body twin. The authors were unable to predict whether a healthy fetus could result from a polar body twinning. In 2003 a study argued that many cases of triploidity arise from semi-identical twinning. In 2007, a study reported a case of a pair of living twins, one intersex and one a phenotypical male. The twins were both found to be chimeras and to share all of their maternal DNA but only half of their father's DNA. 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.
Degree of separation
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.
It is a common misconception that two placentas means twins are dizygotic. But if monozygotic twins separate early enough, the arrangement of sacs and placentas in utero is indistinguishable from dizygotic twins.
|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 ) and in 18–36% (or around 25%) 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.
|Dichorionic-Diamniotic twins form when splitting takes place by the third day after fertilization.|
|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, and in 0.03% of all pregnancies. Monochorionic-Diamniotic twins are almost always monozygotic, with a few exceptions where the blastocysts have fused.
|Monochorionic-Monoamniotic||When the division of the developing zygote into 2 embryos occurs, 99% of the time it is within 8 days of fertilization
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.|
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.|
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."
From 1980–97, the number of twin births in the United States rose 52%. This rise can at least partly be attributed to the increasing popularity of fertility drugs like Clomid 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.
About 1 in 90 human births (1.1%) results from a twin pregnancy. 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. In Cândido Godói, one in five pregnancies has resulted in twins. 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. 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. A high twinning rate has also been observed in other places of the world, including Igbo-Ora in Nigeria and Kodinji in India.
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.
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.
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.
A 15-year German study of 8,220 vaginally delivered twins (that is, 4,110 pregnancies) in Hesse yielded a mean delivery time interval of 13.5 minutes. 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.
A longer delivery interval can lead to twins being born in different places. In one instance a woman in the north of England gave birth unexpectedly to a baby at home; she was rushed to the nearest hospital, which was in Scotland, where she gave birth to the second twin an hour and a half later. One twin is English, and the other Scottish, although both share British nationality.
Complications of twin pregnancy
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. 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. Also, in an unknown proportion of cases, two zygotes may fuse soon after fertilisation, resulting in a single chimeric embryo, and, later, fetus.
Conjoined twins (or the deprecated term "siamese") 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 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.
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, mystifyingly, was not the mother of two of her three children; the mother was found to be a chimera and those of her cells tested happened to derive from one of the fused twins, while two of the children were derived from eggs derived from cells from the other twin making up their mother.
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
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.
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
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.
Management of twin pregnancies
A study in 2013 involving 106 participating centers in 25 countries came to the conclusion that, in a twin pregnancy of a gestational age between 32 weeks 0 days and 38 weeks 6 days, and the first twin is in cephalic presentation, planned Cesarean section does not significantly decrease or increase the risk of fetal or neonatal death or serious neonatal disability, as compared with planned vaginal delivery. In this study, 44% of the women planned for vaginal delivery still ended up having Cesarean section for unplanned reasons such as pregnancy complications. In comparison, it has been estimated that 75% of twin pregnancies in the United States were delivered by Cesarean section in 2008. Also in comparison, the rate of Cesarean section for all pregnancies in the general population varies between 40% and 14%.
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 are now being supplemented with molecular genetic studies which identify individual genes.
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.
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 different sexes (one male, one female). There can be monozygotic boy/girl twins if the sex gene of the embryo has an extra x chromosome (the fertilized egg would be an xxy) then when the egg splits one can have xx (girl) genes and one can have xy (boy) genes. This is rare, but possible. Records show there are only 4 known cases of these type twins. The probability of this is so vanishingly small 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.
Monozygotic twins can develop differently, due to different genes being activated. 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.
Mirror Image Twins
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, even personality traits connected to right-brain or left-brain dominant individuals. 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.
Language development in twins
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. The reasons for this phenomenon are still in question; however, idioglossia was thought to be the major cause. Idioglossia is defined as a private language that is invented usually by young children, specifically twins. 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. Current research is looking into the impacts of a richer social environment for these twins to stimulate their development of language.
Twins are common in many animal species, such as cats, sheep, ferrets, giant pandas, dogs 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. A female calf that is the twin of a bull becomes partially masculinized and is known as a freemartin.
- MedicineNet > Definition of Twin Last Editorial Review: 6/19/2000
- Michael R. Cummings "Human Heredity Principles and issues" pg 104
- "Twins, Triplets, Multiple Births: MedlinePlus". Nlm.nih.gov. Retrieved 2013-09-16.
- Martin, Joyce A.; Hamilton, Brady E.; Osterman, Michelle J.K. "Three Decades of Twin Births in the United States, 1980–2009" , National Center for Health Statistics Data Brief, No. 80, January 2012
- Zach, Terence; Arun K Pramanik and Susannah P Ford (2007-10-02). "Multiple Births". WebMD. Retrieved 2008-09-29.
- "Genetics or yams in the Land of Twins?". Independent Online. 2007-11-12. Retrieved 2008-09-29.
- "The Land Of Twins". BBC World Service. 2001-06-07. Retrieved 2008-09-29.
- 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. p. 34. ISBN 1-85070-666-2.
- "What’s in a yam? Clues to fertility, a student discovers". 1999. Retrieved 2008-09-29.
- Smits, Jeroen; Christiaan Monden (2011). "Twinning across the Developing World". In Newell, Marie-Louise. PLoS ONE (Public Library of Science) 6 (9): e25239. doi:10.1371/journal.pone.0025239. PMC 3182188. PMID 21980404.
- Elliott, JP (December 2008). "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."
- "Probability of having twins hereditary?". Go Ask Alice!. Retrieved 15 March 2012.
- Fitch, Karen. "Ask a Geneticist". Understanding Genetics. Stanford School of Medicine. Retrieved 15 March 2012.
- "Identical and Fraternal Twins - Determining Zygosity in Twins". Multiples.about.com. 2013-07-16. Retrieved 2013-09-16.
- "United States: People". The World Factbook. Central Intelligence Agency. 2008-09-04. Retrieved 2008-10-02.
- "Italy: People". The World Factbook. Central Intelligence Agency. 2008-09-04. Retrieved 2008-10-02.
- 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.
- Cheng, Maria; Associated Press (2007-07-03). "Study: Twins form after embryo collapses". USA Today. Retrieved 2008-09-30.
- April Holladay (2001-05-09). "What triggers twinning?". WonderQuest. Retrieved 2007-03-22.
- 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.
- 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. Retrieved 2008-10-02.
- "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). European Society of Human Reproduction and Embryology. 2007-07-02. Retrieved 2008-09-30.
- Patwari, P; Lee, RT (2008 Aug 1). "Mechanical control of tissue morphogenesis.". Circulation research 103 (3): 234–43. doi:10.1161/CIRCRESAHA.108.175331. PMID 18669930.
- 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. PMC 1012913. PMID 6007033.
- 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.
- "Prenatal diagnosis of heterokaryotypic mosaic twins discordant for fetal sex.". Prenat Diagn 20 (12): 999–1003. Dec 2000. PMID 11113914.
- "Identical twins are genetically different, research suggests | Mail Online". Dailymail.co.uk. 2012-11-13. Retrieved 2013-09-16.
- 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. PMC 1174919. PMID 16009939.
- 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.[page needed]
- Plomin, Robert (2001). Behavioral genetics. New York: Worth Pubs. ISBN 0-7167-5159-3. OCLC 43894450.[page needed]
- Mandler,G. (2001) Apart from genetics: What makes monozygotic twins similar? Journal of Mind and Behavior,22,147-159.
- Gilbert, Scott F. (2006). "Non-identical Monozygotic Twins". DevBio: A Companion to Developmental Biology (Eighth ed.). Sinauer Associates. Retrieved 2008-02-29.
- Fierro, Pamela Prindle. "Polar body twinning". About.com. Retrieved 2008-02-29.
- "FAQ - Twin Zygosity". Proactive Genetics. Retrieved 2008-02-29.
- Fierro, Pamela Prindle. "Semi-Identical Twins - New Twin Type - SemiIdentical Type of Twinning". About.com. Retrieved 2008-02-29.
- Bieber, Frederick R.; Walter E. Nance; Cynthia C. Morton; Judith A. Brown; Fay O. Redwine; Robert L. Jordan and T. Mohanakumar (1981-08-14). "Genetic Studies of an Acardiac Monster: Evidence of Polar Body Twinning in Man". Science (American Association for the Advancement of Science) 213 (4509): 775–777. doi:10.1126/science.7196086. JSTOR 1686613. PMID 7196086. Non-identical Monozygotic Twins Lay summary – DevBio: A Companion to Developmental Biology (2006).
- Golubovsky, M.D. (02 2003). "Postzygotic diploidization of triploids as a source of unusual cases of mosaicism, chimerism and twinning". Human Reproduction (Oxford Journals) 18 (2): 236–242. doi:10.1093/humrep/deg060. PMID 12571155. Retrieved 2008-02-29. 'Semi-identical' twins discovered Lay summary – Nature News (2007-03-26).
- 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 (04 2007). "A case of true hermaphroditism reveals an unusual mechanism of twinning". Human Genetics (Berlin / Heidelberg: Springer) 121 (2): 179–185. doi:10.1007/s00439-006-0279-x. ISSN 1432-1203. PMID 17165045. Retrieved 2008-02-29. Semi-identical twins discovered Lay summary – BBC News (2007-02-27).
- Shulman, Lee S.; Vugt, John M. G. van (2006). Prenatal medicine. Washington, DC: Taylor & Francis. p. Page 447. ISBN 0-8247-2844-0.
- Curran, Mark (2005-11-02). "Twinning". Focus Information Technology. Retrieved 2008-10-10.
- 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.
- 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.
- Pregnancy-Info -- > Monoamniotic Twins Retrieved on July 9, 2009
- MoMo Twins; Monochorionic Monoamniotic Twins By Pamela Prindle Fierro, About.com. Retrieved on July 9, 2009
- 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. Retrieved 2008-09-29.
- 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. Retrieved 2008-09-29.
- Asch, Richard H.; John Studd (1995). Progress in Reproductive Medicine Volume II. Informa. ISBN 1-85070-574-7. ISSN 1358-8702. OCLC 36287045. [page needed]
- 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.
- Nick Evans (21 January 2009big). "Nazi Angel of death Josef Mengel created twin town in Brazil". The Telegraph.
- Caroline Iggulden (23 January 2009). "The twins from Brazil". The Sun.
- Linda Geddes: Nazi 'Angel of Death' not responsible for town of twins New Scientist online, 27 January 2009
- "The Land Of Twins". BBC World News. 7 June 2001. Retrieved 2009-08-06.
- "Seeing double: the village in deepest Kerala where twins have taken over". The Independent. 12 May 2009. Retrieved 2009-08-06.
- 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.
- Yoruba Customs and Beliefs Pertaining to Twins. Randafricanart.com. Retrieved on 2013-08-10.
- Kulkarni, A. D.; Jamieson, D. J.; Jones, H. W.; Kissin, D. M.; Gallo, M. F.; MacAluso, M.; Adashi, E. Y. (2013). "Fertility Treatments and Multiple Births in the United States". New England Journal of Medicine 369 (23): 2218. doi:10.1056/NEJMoa1301467.
- "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.
- 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.
- BBC: Twins Dylan and Hannah born in England and Scotland, 9 July 2012
- 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.
- New scientist magazine: The stranger within. 15 November 2003, p34
- Barrett, J. F. R.; Hannah, M. E.; Hutton, E. K.; Willan, A. R.; Allen, A. C.; Armson, B. A.; Gafni, A.; Joseph, K. S.; Mason, D.; Ohlsson, A.; Ross, S.; Sanchez, J. J.; Asztalos, E. V.; the Twin Birth Study Collaborative Group (2013). "A Randomized Trial of Planned Cesarean or Vaginal Delivery for Twin Pregnancy". New England Journal of Medicine 369 (14): 1295–1305. doi:10.1056/NEJMoa1214939. PMID 24088091.
- Lee, H. C.; Gould, J. B.; Boscardin, W. J.; El-Sayed, Y. Y.; Blumenfeld, Y. J. (2011). "Trends in Cesarean Delivery for Twin Births in the United States". Obstetrics & Gynecology 118 (5): 1095. doi:10.1097/AOG.0b013e3182318651.
- Women can choose Caesarean birth - BBC article by James Gallagher. 2011.
- Lovell, Tammy (2008-07-17). "Pictured: Proud parents show off their million-to-one black and white twins". Daily Mail (Associated Newspapers Ltd). Retrieved 2008-10-02.
- Moorhead, Joanna (2011-09-24). "Black and white twins". The Guardian. Retrieved 2012-04-07.
- 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. PMC 1013354. PMID 944787. Retrieved 2008-09-29.
- Gilbert, Scott F. (2006). "Non-identical Monozygotic Twins". Developmental biology. Sunderland, Mass: Sinauer Associates. ISBN 0-87893-250-X. OCLC 172964621. Retrieved 2008-09-30.
- 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.
- LiveScience Staff (2007-03-26). "Rare Semi-Identical Twins Discovered". Imaginova. Retrieved 2008-10-01.
- Whitfield, John (2007). "'Semi-identical' twins discovered". Nature. doi:10.1038/news070326-1.
- Twins genetics, details of environmental impacts. Proactivegenetics.com. Retrieved on 2013-08-10.
- Thorpe, K (June 2006). "Twin children's language development". Early Human Development 82 (6): 387–395.
- Usher, L (13). "Idioglossia". Times Literary Supplement: 24.
- Hayashi, C (2007). "Relationship between Twin Language, Twins' Bond, and Social Competence". Twin research and human genetics.
- Cyranoski, D (2009). "Developmental biology: Two by two". Nature 458 (7240): 826–9. doi:10.1038/458826a. PMID 19370006.
- 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 (3): 181–187. JSTOR 1536114.
- 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; Rieteke Van Zalen-Sprock, Pieter Hummel, Gerard Pals, John Van Vugt, Hans Van Der Harten, Yvonne Heins and Kamlesh Madan (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.
- Wenk RE, Houtz T, Brooks M, Chiafari FA (1992). "How frequent is heteropaternal superfecundation?". Acta geneticae medicae et gemellologiae 41 (1): 43–7. PMID 1488855.
- Girela, Eloy; Jose A. Lorente, J. Carlos Alvarez, Maria D. Rodrigo, Miguel Lorente and Enrique Villanueva (1997). "Indisputable double paternity in dizygous twins". Fertility and Sterility 67 (6): 1159–61. 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. 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. doi:10.1073/pnas.0605875104. PMC 1904168. PMID 17576931. Retrieved 2008-10-06.
- Schein, Elyse; Paula Bernstein (2007). Identical Strangers: A Memoir of Twins Separated and Reunited. New York: Random House. ISBN 1-4000-6496-1. OCLC 123390922.
- Helle, Samuli; Virpi Lummaa and 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. Retrieved 2008-10-02.
- "TWINS Guide to the First Year" (PDF). Fort Collins, Colorado: TWINS Magazine. 2008. Retrieved 2008-10-06.
- Samson, Jennifer. "Facts About Multiples: An Encyclopedia of Multiple Birth Records". Retrieved 2008-10-18.
- 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.