Twins in animal biology is a form of multiple birth in which the mother gives birth to two offspring from the same pregnancy, some of the same sex, others of the opposite. There are estimated to be approximately 125 million twins and triplets in the world, and just 10 million identical twins. Triplets refers to three offspring from the same pregnancy. The general term for more than one offspring from the same pregnancy is multiples. A fetus alone in the womb is called a singleton. 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 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 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. 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, "polar body twins", is a phenomemon that was hypothesised to occur and may recently have been proven to exist. Polar body twinning would result in "half-identical" twins.