Twins

Twins in animal biology is a case of multiple birth in which the mother gives birth to two offspring from the same pregnancy, some of the same gender, others of opposite. There are estimated to be around 125 million Twins and Triplets in the world, and just 10 million Identical twins.

Human twins are two individuals who have shared the uterus during a single pregnancy and are usually, but not necessarily, born in close succession. A fetus alone in the womb is called a singleton. Due to the limited size of the mother'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 five variations of twinning that occur commonly in the world. 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.

Another variety of twins, "polar body twins", is a phenomemon that is hypothesized to occur but has not been proven. Polar body twinning would result in "half-identical" twins.

Twins 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.