Twins are two offspring resulting from the same pregnancy, either of the same or opposite sex.

The general term for more than one offspring from the same pregnancy (multiple birth) 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.

Twins are usually, 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.

Twins can either be monozygotic (MZ, colloquially, "identical") or dizygotic (DZ, colloquially, "fraternal"). 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 monozygotic twins (roughly 0.2% of the world population and 8% of all twins). The current rate in the United States is 31 twin births per 1,000 women.

The Yoruba, a large west African ethnic group, have the highest rate of twinning in the world at 45 twins per 1000 live births. Some researchers have claimed this may be because of high consumption of a specific type of yam, dioscorera rotundata or white yam containing a natural hormone phytoestrogen which may stimulate the ovaries to produce an egg from each side.

Types of twins

There are five common variations of twinning. The three most common variations are all dizygotic:

• male-female twins are the most common result, at about 40 percent of all twins born
• female DZ twins (sometimes called sororal twins)
• male DZ twins.

The other two variations are monozygotic twins:

• female MZ twins
• male MZ twins (least common).

Among non-twin births, male singletons are slightly (about five percent) more common than female singletons. There is also the mirror image variations: This is where the twins develop reverse asymmetric features. About 25% of monozygotic twins are mirror image twins. The rates for singletons vary slightly by country. For example, the sex ratio of birth in the US is 1.05 males/female, while it is 1.07 males/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, "polar body twins," is a phenomenon that was hypothesized to occur and may recently have been proven, very rarely, to exist. This would occur when a portion of a mature egg separates itself from the egg. This is known as the first polar body, and it carries all the same genetic information as the egg. If polar body twins are fact, they would occur when two sperm fertilize both the egg and the first polar body. Generally the first polar body disintegrates. Polar body twinning would result in "half-identical" twins.