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. Since some premature births often have health consequence to the babies, twins birth are more often handled with special procedures than regular births. This article is limited to this usage of the term "twin".
Fraternal twins (commonly known as "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, and these twins are therefore also known as dizygotic.
Studies show that there is a genetic basis for fraternal twinning—that is, non-identical twins do run in families.
Identical twins occur when a single egg is fertilized to form one zygote (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 blood 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; they are only usually, but not necessarily, the same gender. They look alike, except that they are sometimes mirror images, but examination of details such as fingerprints can tell them apart. As they mature, identical twins often become less alike because of lifestyle choices or external influences such as scars.
It is thought that identical twins do not run in families, but rather identical twins occur more or less randomly.
Identical twins can behave as differently as any other siblings (a matter of much interest to psychologists). They develop their own individual personalities to enable themselves to be identified as individual persons. Many identical twins spend most of their time together (especially as children), so people can assume that they will behave alike just as they look alike; however, this is not necessarily the case. Twins establish their own individual likes and dislikes. This is not to say they are totally different, but there are usually obvious signs of differences when the identical twins are observed separately or together.
In extremely rare cases, identical twins have been born with opposite sexes (one male, one female). In these cases, identical twin boys are conceived, but, during the twinning process, one twin loses a Y chromosome (boys have chromosome type XY while girls have XX). Without a Y chromosome to trigger the production of male sex hormones , this fetus develops as a girl by default, but a girl with only one X chromosome (chromosome type XO). The co-twin is unaffected, and develops as a boy as normal.
As a result of her genetic abnormality, the girl will suffer from Turner syndrome, which is distinguishable by short stature, folds of skin at the neck, abnormal development of secondary sexual characteristics, and an intellectual deficit known as space-form blindness . Turner syndrome can occur in any birth (including singletons, fraternal female twins, or identical female twins), so an individual with Turner syndrome is not automatically a twin. Turner syndrome occurs in about 1 in 10,000 of all births, but cases of mixed sex identical twins are much rarer—only three cases have been documented.
If a mother's egg splits before fertilization, and then both go on to be fertilized, it can result in semi-identical twins, who would be identical on their mother's side but fraternal on the father's side.
Main article: Vanishing twin
Researchers suspect that more pregnancies start out as multiples than come to term that way. Early obstetric ultrasonography exams sometimes reveal an "extra" fetus, which fails to develop and instead disintegrates and vanishes.
Main article: Conjoined twin
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.
Main article: Parasitic twin
Sometimes one twin fetus will fail to develop completely and continue to cause problems for its surviving twin. One fetus acts as a parasite towards the other.
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% of all births. Thus, approximately 6% of children born in the US in 2001 were twins.
Main article: Multiple birth
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.
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 when the following factors are present in the woman:
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).
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.
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.