Twins are a form of multiple birth in which the mother gives birth to two offspring from the same pregnancy, either of the same or opposite sex.The general term for more than one offspring from the same pregnancy is multiples, for example triplets refers to cases of three offspring from the same pregnancy. 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 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).Twins can either be monozygotic or dizygotic (identical or fraternal).