Twins are two offspring produced by the same pregnancy. Twins can either be monozygotic , meaning that they can develop from just one zygote that will then split and form two embryos, or dizygotic , meaning that they can develop from two different eggs, each are fertilized by separate sperm cells.
In contrast, a fetus which develops alone in the womb is called a singleton, and the general term for one offspring of a multiple birth is multiple.
The twin birth rate in the United States rose 76% from 1980 through 2009, from 18.9 to 33.3 per 1,000 births. The Yoruba have the highest rate of twinning in the world, at 45–50 twin sets per 1,000 live births, possibly because of high consumption of a specific type of yam containing a natural phytoestrogen which may stimulate the ovaries to release an egg from each side.
In Central Africa there are 18–30 twin sets per 1,000 live births. In Latin America, South Asia, and Southeast Asia, the lowest rates are found; only 6 to 9 twin sets per 1,000 live births. North America and Europe have intermediate rates of 9 to 16 twin sets per 1,000 live births.
Multiple pregnancies are much less likely to carry to full term than single births, with twin pregnancies lasting on average 37 weeks, three weeks less than full term. (source : wikipedia)