How often do twin births occur?

In the US, there were 135,336 births in twin deliveries in 2014, the most recent year for which government statistics are available. This represents an increase from 2013, but is lower than the peak years of 2006-09. In Washington State, there were 2,660 births in twin deliveries in 2014.

Percentage of twin births by county, Washington State, 2014

The birth rate for identical (monozygotic, or MZ) twins is the same throughout the world, at about 4 per 1000 births. It does not seem to be related to where people live, what they eat, their racial/ethnic background, or any other factors. This suggests that identical twinning is a random event that is not influenced by genes or environment.

But the birth rates of fraternal (dizygotic or DZ) twins vary by population throughout the world. The highest DZ twinning rate is found among black populations in Africa, and it varies among tribes. The Yoruba tribe in the West African country of Nigeria has an overall twinning rate of 45 per 1000, and 93% of those twins are DZ. This may be related to their diet, which is high in a particular type of yam. This yam contains an estrogen-like substance that may stimulate hyperovulation, which is the tendency to release multiple eggs during ovulation. DZ twins occur when two eggs are released at the same time, and are fertilized by two different sperm.

The DZ twinning rate among African-Americans is higher than among American whites. The lowest rates of DZ twinning are found in Asian populations, where it is about 4 per 1000 births.

DZ twinning seems to run in some families, suggesting that there is a genetic factor causing hyper-ovulation in some women. The racial differences in DZ twinning also suggest the possibility of genetic factors.

The overall rate of twinning in the US has increased dramatically over the last 30 years. In 1980 the rate was 19 per 1000 births. In 2009 the rate had increased to 33.9 per 1000 births. The increase is entirely due to an increase in DZ twin births.