The combined ratio of monozygotic and dizygotic twins to singletons is approximately 1 to 100. For monozygotic twins, the ratio is the same regardless of ethnicity. However for dizygotic twins, ethnicity is thought to have an effect: for example the ratio is considered to be between 15 and 20 to 1000 in the Nordic countries but only between 6 and 10 to 1000 in Japan. In recent years, the ratio of dizygotic twins has been growing due to the increased use of assisted reproductive technology.

The likelihood of birthing twins is also thought to be influenced by both genes and the environment.