Twins are two offspring from the same pregnancy, classified as either monozygotic (identical), originating from a single split zygote, or dizygotic (fraternal), originating from two separately fertilized eggs. The human twin birth rate in the United States rose dramatically by 76% between 1980 and 2009, while the Yoruba people of West Africa hold the world's highest twinning rate, possibly due to their consumption of a specific yam containing a natural phytoestrogen. Fraternal twinning is influenced by fertility treatments, maternal age, and a genetically linked tendency to hyper-ovulate, but identical twinning has no known genetic cause. Twin pregnancies usually last only 37 weeks on average, shorter than the typical 40-week singleton pregnancy. Interestingly, while female-male twins are the most common pairing, female twins are actually more common than male twins overall, as male fetuses have a higher in-utero mortality rate.