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. 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. There is no known genetic link for identical twinning. Other factors that increase the odds of having fraternal twins include maternal age, fertility drugs and other fertility treatments, nutrition, and prior births. Some women intentionally turn to fertility drugs in order to conceive twins. Types and zygosity
The vast majority of twins are either dizygotic (fraternal) or monozygotic (identical). Less common variants are discussed further down the article. Fraternal twins can be any of the following:

Female–female twins: Sometimes called sororal twins (25%). Male–male twins: Sometimes called fraternal twins (25%). Female-male twins: This is the most common pairing (50%), simply by virtue of it encompassing both "female-male" (25%) and "male-female" (25%) twins.Among non-twin births, male singletons are slightly (about five percent) more common than female singletons. The rates for singletons vary slightly by country. For example, the sex ratio of birth in the US is 1.05 males/female, while it is 1.07 males/female in Italy.
Drug Information Portal. U.S. National Library of Medicine.