The term twin refers to two individuals or one of two individuals who have shared the same uterus (womb), usually born on the same day but not necessarily. There are two different kinds of twins: identical and fraternal.

Identical twins occur when a single egg is fertilized and then divides into two separate embryos. The two embryos develop into fetuses sharing the same womb, and generally the same amnion. These monozygotic twins are identical genetically unless there has been a mutation in development, and are necessarily the same gender.

Fraternal 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. These dizygotic twins are no more similar genetically than any siblings and develop in separate amnions, with separate placentae. They may be of mixed gender or the same gender.

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

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.

A fetus alone in the womb is called a singleton.

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 Twinning rate at a small village in south Brazil). 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 percent of all births. Thus, over 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 percent chance of delivering monozygotic twins!) Some evidence suggests that the environment of the womb causes the zygote to split in most cases.

Dizygotic pregancies 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%.

Twinning in animals

Multiple births are common in many animal species, such as cat, sheep, and ferret. 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.

List of twins

Topics related to twin pregnancy

  • Pre-term labor
  • Twin to Twin Transfusion Syndrome
  • Disappearing twins

Twin could also refer to a doppelganger or clone

Twin is a link between two towns, cities, etc. where the councils of each place interact. Usually the twins are in different countries.

Twin can refer to anything in a pair, whether a shoe, or a scientific study, except when the objects have specific differences-such as a cup and a saucer.