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| ===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 |
] 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== | ==Human twins== | ||
Revision as of 10:37, 8 June 2004
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. A fetus alone in the womb is called a singleton.
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 be of mixed gender 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 ). 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 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.
Sometimes a 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 of its twin creating a fetus in fetu, which continues to parasitize its host outside the womb.
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
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, e.g. Twin Towers, shoes, scientific studies, etc., except when the objects have specific differences, such as a cup and a saucer.