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However, males are also more susceptible than females to die in utero, and since the death rate in utero is higher for twins, it leads to female twins being more common than male twins. For example, the sex ratio of birth in the US is 1.05 males/female, while it is 1.07 males/female in Italy. The rates for singletons vary slightly by country.
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Other factors that increase the odds of having fraternal twins include maternal age, fertility drugs and other fertility treatments, nutrition, and prior births. There is no known genetic link for identical twinning. Women who have a family history of fraternal twins have a higher chance of producing fraternal twins themselves, as there is a genetically linked tendency to hyper- ovulate.
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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. North America and Europe have intermediate rates of 9 to 16 twin sets 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. In Central Africa, there are 18–30 twin sets (or 36–60 twins) per 1,000 live births. The Yoruba people have the highest rate of twinning in the world, at 45–50 twin sets (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. 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. 9.2 Monozygotic twins of different sexes.2.3 Polar body and semi-identical twins.
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2.2.3 Genetic and epigenetic similarity.