Twins are two offspring produced by the same pregnancy. Twins can be either monozygotic ('identical'), meaning that they develop from one zygote, which splits and forms two embryos, or dizygotic ('non-identical' or 'fraternal'), meaning that each twin develops from a separate egg and each egg is fertilized by its own sperm cell. In rare cases twins can have the same mother and different fathers (heteropaternal superfecundation). In contrast, a fetus that develops alone in the womb (the much more common case, in humans) is called a singleton, and the general term for one offspring of a multiple birth is a multiple. Unrelated look-alikes whose resemblance parallels that of twins are referred to as doppelgangers. Statistics The human twin birth rate in the United States rose 76% from 1980 through 2009, from 9. 4 to 16. 7 twin sets (18. 8 to 33. 3 twins) per 1, 000 births. The Yoruba people have the highest rate of twinning in the world, at 4550 twin sets (90100 twins) 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 1830 twin sets (or 3660 twins) per 1, 000 live births. In North America, South Asia (India, Pakistan, Bangladesh, Nepal), 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.
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