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The term twin can be used to refer to a pair of any similar objects or concepts, such as the Twin Towers, twin primes, two shoes, or two scientific studies. The term twin would not usually be used for objects that have specific differences, such as a cup and a saucer. One specific instance is the 'twinning' of two towns or cities (usually in different countries) where the councils and the general population in each place become formally associated with one another: for example, exchange visits may be arranged. The term twin can also refer to a doppelganger or clone.

The term twin also specifically refers to two individuals (or one of two individuals) who have shared the same uterus (womb) and 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. Since some pre-term labor often have health consequence to the babies, twins birth are often handled with special procedures than regular births. The discussion below is limited to this usage of the term twin.

Types of twins

Identical twins

Identical twins occur when a single egg is fertilized to form one zygote (so identical twins are monozygotic) but the zygote then divides into two separate embryos. The two embryos develop into fetuses sharing the same womb. 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.

Sharing the same amnion (or the same amnion and placenta) 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. Monochorionic twins, sharing one placenta, usually also share the placental bood supply. In rare cases, 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 are necessarily the same gender. When such twins grow up, they will look physically identical (barring certain variables, such as weight gain).

It is thought that identical twins do not run in families, but rather identical twins occur more or less randomly.

Fraternal twins

Fraternal twins (that is, non-identical twins) occur when two fertilized eggs are implanted in the uterine wall at about the same time, within the same menstrual cycle, or in rare cases within one menstrual cycle of each other. The two eggs form two zygotes, so these twins are also known as dizygotic.

Dizygotic twins are no more similar genetically than any siblings and develop in separate amnions, with separate placentae. They may have different genders or the same gender.

Studies show that there is a genetic basis for fraternal twinning—that is, non-identical twins do run in families.

Conjoined twins

Conjoined twins are monozygotic twins whose bodies are joined together at birth. This occurs where the single zygote of identical twins fails to separate completely. This condition occurs in about 1 in 100,000 pregnancies.

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 [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, about 6% of children born in the US in 2001 were twins.

Twin studies refers to the practice of assessing identical twins for medical, genetic, or psychological studies to try to find innate similarities and differences. Twins that have been separated early in life and raised in separate households are the most sought-after for these studies.

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 (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.

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.

Dizygotic twin pregnancies are slightly more likely in women of African descent, in women between 30 and 40 years old, in tall and heavy women, in women who conceive soon after stopping birth control pills, and in women who have had several previous pregnancies. Women who have been taking fertility drugs have the greatest chance of multiple births, 10-20%.

Researchers suspect that more pregnancies start out as multiples than come to term that way. Early obstetric ultrasound exams sometimes reveal an "extra" fetus, which fails to develop and instead disintegrates and vanishes. This is sometimes known as a disappearing twin.

Sometimes one twin fetus will fail to develop completely and continue to cause problems for its surviving twin and as such is known as a parasitic twin. Extremely rarely, a fetus can be trapped inside its twin creating a fetus in fetu that continues to parasitize its host outside the womb.

Twinning in animals

Multiple births are 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.

See also