Young Brunette Twin Girls With White Stockings At Home

The twin birth rate in the United States is slightly above 32 twin live births per 1,000 live births, while the Yoruba have the highest rate of twinning in the world, at 45 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.
Due to the limited size of the mother's womb, multiple pregnancies are much less likely to carry to full term than single births, with twin pregnancies lasting only 37 weeks (3 weeks less than full term) on average.