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Main article: The psychology of twins

Fraternal twin boys in the tub

Twins in animal biology is a case of multiple birth in which the mother gives birth to two offspring from the same pregnancy.

Human twins are two individuals who have shared the uterus during a single pregnancy and are usually, but not necessarily, born in close succession. A fetus alone in the womb is called a singleton. Due to the limited size of the mother's womb, multiple pregnancies are much less likely to carry to full term than singleton births, with twin pregnancies usually lasting around 34 to 36 weeks. Since premature births can have health consequences for the babies, twin births are often handled with special precautions.

Twinning is common in many animal species, such as cats, sheep, and ferrets. 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.

Note: Sometimes multiple births may involve more than two fetuses. See multiple birth for more information.

Types of twins

Fraternal twins

Main article: Heterozygotic twins

Heterozygotic twins or Fraternal twins (commonly known as "non-identical twins") usually occur when two fertilized eggs are implanted in the uterine wall at the same time. The two eggs form two zygotes, and these twins are therefore also known as dizygotic as well as "biovular" twins. When two eggs are independently fertilized by two different sperm cells, fraternal twins result.

Dizygotic twins, like any other siblings, have a very small chance of having the exact same chromosome profile, but most likely have a number of different chromosomes that distinguish them. Like any other siblings, fraternal twins may look very similar, particularly given that they are the same age. However, fraternal twins may also look very different from each other. They may be a different sex or the same sex. Mixed-race twins, or twins born to parents of mixed racial origin, can vary considerably in their skin colouration and other features.

Studies show that there is a genetic basis for fraternal twinning. However, it is only the female partner that has any influence on the chances of having fraternal twins as the male cannot make her release more than one ovum. Fraternal twinning ranges from 1 or 2 per thousand births in Japan (similar to the rate of identical twins) to 14 and more per thousand in some African states.[citation needed]

Fraternals are also more common in women of a "certain age," with twinning rates doubling in mothers over the age of 35.[citation needed] Of course, with the advent of technologies and techniques to assist women in getting pregnant, the rate of fraternals has increased markedly, most notably on New York City's Upper East Side. In 1995, there were 3,707 twin births in the city; in 2003, there were 4,153; and in 2004, there were 4,655. Triplet births have also risen, from 60 in 1995, to 299 in 2004. (New York Times 3/3/2006).

Identical twins

Main article: Monozygotic twins

Monozygotic twins or Identical twins occur when a single egg is fertilized to form one zygote (monozygotic) which then divides into two separate embryos. The two embryos develop into fetuses sharing the same womb. When one egg is fertilized by one sperm cell, and then divides and separates, two identical cells will result. Depending on the stage at which the zygote divides, identical twins may share the same amnion (in which case they are known as monoamniotic) or not (diamniotic). Diamniotic identical twins may share the same placenta (known as monochorionic) or not (dichorionic). All monoamniotic twins are monochorionic. Also note that any monochorionic or monoamniotic twins are identical twins. This condition does not occur in fraternal twins.

The later in pregnancy that twinning occurs, the more structures will be shared. Zygotes that twin at the earliest stages will be diamniotic and dichorionic ("di-di"). Twinning between 4 to 8 days after fertilization typically results in monochorionic-diamniotic ("mono-di") twins. Twinning between 8 to 12 days after fertilization will usually result in monochorionic-monoamniotic ("mono-mono") twins. Twinning after 12 days post-fertilization will typically result in conjoined twins.

In placental species, twins sharing the same amnion (or the same amnion and placenta/chorion) can cause complications in pregnancy. For example, the umbilical cords of monoamniotic twins can become entangled, reducing or interrupting the blood supply to the developing fetus. About 50% of mono-mono twins die from umbilical cord entanglement. Monochorionic twins, sharing one placenta, usually also share the placental blood supply. These twins may develop such that blood passes disproportionately from one twin to the other through connecting blood vessels within their shared placenta, leading to twin-to-twin transfusion syndrome.

Monozygotic twins are genetically identical (unless there has been a mutation in development) and they are the same gender. (On extremely rare occasions, an original XXY zygote may form monozygotic boy/girl twins by dropping the Y chromosome for one twin and the extra X chromosome for the other.) Monozygotic twins generally look alike. Fine physical details such as fingerprints will differ. As they mature, identical twins often become less alike because of lifestyle choices or external influences.

The likelihood of a single fertilisation resulting in identical twins is a random event, not a hereditary trait, and is uniformly distributed in all populations around the world. This is in marked contrast to fraternal twinning which ranges from 1 or 2 per thousand births in Japan (similar to the rate of identical twins) to 14 and more per thousand in some African states[citation needed] The exact cause for the splitting of a zygote or embryo is unknown.

Studies have shown that identical twins reared in different environments share similar personality traits, mannerisms, job choices, attitudes, and interests. These findings add to the belief that many of our behaviors are derived from our genes.

Identical twins have 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 3 to 74 showed that the youngest twins have relatively few epigenetic differences. The number of differences between identical twins increases with age. 50-year-old twins had over three times the epigenetic difference of 3-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. (Fraga, et al., 2005).

Mirror twins

Some monozygotic twins are known as "mirror twins" or mirror image twins. These are identical twins with opposite features, that is, one may be right-handed and the other may be left-handed; hair will whorl in the opposite direction, and so on. Mirror twinning is comparatively rare in humans. It results from a late split of the fertilized egg at around 9-12 days. One mirror may or may not have situs inversus. This is where some or all of the organs will be on the opposite side of the body, such as the heart being on the right (dextrocardia). Such conditions are usually associated with a higher incidence of other birth defects.

Complications of twin pregnancy

A chimera is an ordinary person or animal except that some of his or her parts actually came from his or her twin. A chimera may arise either from identical 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 colouration in human chimeras. A chimera may be a hermaphrodite, composed of cells from a male twin and a female twin.

Human twins

Historically, about 1 in 80 human births (1.25%) has been the result of a twin pregnancy. The rate of twinning varies greatly among ethnic groups, ranging as high as about 6% for the Yoruba or 10% for a tiny Brazilian village.[1] 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. Thus, approximately 6% of children born in the US in 2001 were twins.

Nevertheless, the rate of identical twins remains at about 1 in 250 across the globe, further suggesting that pregnancies resulting in identical twins occur randomly.

Predisposing factors

The cause of monozygotic twinning is unknown.

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

  • She is of African descent (especially Yoruba)
  • Between the age of 30 and 40 years
  • Greater than average height and weight
  • Several previous pregnancies.

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, allowing the possibility of multiples. Many fertility treatments have no effect on the likelihood of multiple births.

There is also speculation that the West African predisposition to twinning is due to the large amount of yams in their diet. These yams contain phytoestrogen, this natural hormone may stimulate the ovaries.

Human twin studies

Main article: Twin study

Twin studies are studies that assess identical (monozygotic) twins for medical, genetic, or psychological characteristics to try to isolate genetic influence from 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 invaluable in the exploration of human nature.

Unusual twinnings

There are some patterns of twinning that are exceedingly rare: while they have been reported to happen, they are so unusual that most obstetricians or midwives may go their entire careers without encountering a single case.

Among fraternal twins, in rare cases, the eggs are fertilised 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). 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.

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) that multiples having different genders 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 genders 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.

See also


  1. U. Matte et al. Study on possible increase in twinning rate at a small village in south Brazil. Acta Genet Med Gemellol (Roma). 45(4), 1996. 431–437.


  1. Fraga MF, Ballestar E, Paz MF, Ropero S, Setien F, Ballestar ML, Heine-Suner D, Cigudosa JC, Urioste M, Benitez J, Boix-Chornet M, Sanchez-Aguilera A, Ling C, Carlsson E, Poulsen P, Vaag A, Stephan Z, Spector TD, Wu YZ, Plass C, Esteller M (2005). Epigenetic differences arise during the lifetime of monozygotic twins. Proceedings of the National Academy of Sciences 102 (30): 10413–10414. PMID 16009939.
  2. Nieuwint A, Van Zalen-Sprock R, Hummel P, Pals G, Van Vugt J, Van Der Harten H, Heins Y, Madan K. (1999). 'Identical' twins with discordant karyotypes. Prenatal Diagnosis 19 (1): 72-6. PMID 10073913.
  3. 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.
  4. Girela E, Lorente JA, Alvarez JC, Rodrigo MD, Lorent M, Villaneuva E (1997). Indisputable double paternity in dizygous twins. Fertility and Sterility 67 (6): 1159-61. PMID 9176461.
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