Twin

The term twin most notably refers to two individuals (or one of two individuals) who have shared the same uterus (womb) and are usually, but not necessarily, born on the same day. A fetus alone in the womb is called a singleton. Due to the limited size of the mother's womb, multiple pregnancy is much less likely to carry to full term than singleton birth (twins usually around 34 to 36 weeks). Since some premature births often have health consequence to the babies, twins birth are more often handled with special procedures than regular births.

Human twins

Historically, about 1 in 80 human births (1.2%) 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 (see http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?holding=npg&cmd=Retrieve&db=PubMed&list_uids=9181177&dopt=Abstract). 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.

Related Topics:
Ethnic group - Yoruba - Multiple birth - 2001 - US

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

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Multiple births

Main article: Multiple birth

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Sometimes multiple births may involve more than two fetuses. If there are three, they are called triplets; four, quadruplets; five, quintuplets; six, sextuplets, seven, septuplets, and so on. Before the advent of ovulation-stimulating drugs, triplets were quite rare (approximately 1 in 8000 births) and higher order births so rare as to be almost unheard of. Multiple pregnancies are usually delivered before the full term of 40 weeks gestation: the average length of pregnancy is around 37 weeks for twins, 34 weeks for triplets and 32 weeks for quadruplets.

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Predisposing factors

The cause of monozygotic twinning is unknown. Fewer than 20 families have been described with an inherited tendency towards monozygotic twinning (people in these families have nearly a 50% chance of delivering monozygotic twins). Some evidence suggests that the environment of the womb causes the zygote to split in most cases.

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

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• She is of African descent
• Between the age of 30 and 40 years
• Greater than average height and weight
• Conception is soon after the cessation of oral contraceptives
• Several previous pregnancies.

Women who have been taking fertility drugs have the greatest chance of multiple births, ranging from 10 to 20 percent. This is primarily down to the passing of multiple embryos into the uterus in order to increase the success rate of in vitro fertilisation (IVF).

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Twin studies

See main article at twin study

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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.

Related Topics:
Medical - Genetic - Psychological - Environment - Human nature

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Twin Studies currently recruiting subjects

The following Twin Studies are ongoing studies that are recruiting subjects:

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• The Maudsley Bipolar Twin Study

Unusual Twinnings

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

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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.

Related Topics:
Sexual intercourse - Menstrual cycle - Superfecundation - Pregnancy - Superfetation

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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.

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