Human twins are two individuals who have shared the uterus during a single pregnancy and are usually, but not necessarily, born in close succession. Due to the limited size of the motherâ€™s womb, multiple pregnancies are much less likely to carry to full term than singleton births, with twin pregnancies lasting only 37 weeks on average, 3 weeks less than full term. Since premature births can have health consequences for the babies, twin births are often handled with special precautions.
There are estimated to be approximately 125 million human twins and triplets in the world (roughly 1.9% of the world population), and just 10 million identical twins (roughly 0.2% of the world population and 8% of all twins).
Here are 5 variations of twinning that commonly occur.
The three most common variations are all fraternal:
(1) male-female twins are the most common result, at about 40% of all twins born; (2) female fraternal twins (sometimes called sororal twins); (3) male fraternal twins. The last two are identical: (4) female identical twins and (5) (least common) male identical twins. Male singletons are slightly, about 5%, more common than female singletons. The rates for singletons vary slightly by country as shown in the CIA World Fact book. For example, the sex ratio at birth in the US is 1.05 male(s)/female where it is 1.07 male(s)/female in Italy. However, males are also more susceptible than females to death in utero, and since the death rate in utero is higher for twins, it leads to female twins being more common than male twins.
Another variety of twins, â€œpolar body twins,â€ (one egg fertilized by two different sperm) is a phenomenon that was hypothesized to occur and may recently have been proven to exist. Polar body twinning would result in â€œhalf-identicalâ€ twins.