<P> 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 ("fraternal"), meaning that they develop from two different eggs . In fraternal twins, each twin is fertilized by its own sperm cell . </P> <P> In contrast, a fetus that develops alone in the womb is called a singleton, and the general term for one offspring of a multiple birth is multiple . Non-related look - alikes whose resemblance parallels that of twins are referred to as doppelgangers . </P> <P> The human twin birth rate in the United States, rose 76% from 1980 through 2009, from 18.9 to 33.3 per 1,000 births (about 80%) The Yoruba have the highest rate of twinning in the world, at 45--50 twin sets (or 90--100 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 . </P> <P> In Central Africa there are 18--30 twin sets (or 36--60 twins) 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 . </P>

How many sets of twins are there in the us
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