How Often Do Twin Births Occur?

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. Since 2009, twin births have been declining. In 2021, the most recent year for which government statistics are available, there were 114,161 twin births in the US, and 2,384 twin births in Washington State.

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.