The frequency of twins depends on the type of twins. Identical twins occur about once in every 250 births around the world. This type of twin formation appears to be uninfluenced by age, race, heredity, number of pregnancies or medications taken for infertility (fertility drugs). The incidence of fraternal twins is influenced by race, heredity, maternal age, the number of previous pregnancies and the use of fertility drugs.
The frequency of multiple fetuses varies significantly among different races. Twins occur in 1 out of every 100 pregnancies in white women compared to 1 out of every 79 pregnancies in black women. Certain areas of Africa have an incredibly high frequency of twins. In some places, twins occur once in every 20 births! The occurrence of twins among Asians is less common: about 1 in every 150 births.
Heredity also plays a part in the occurrence of twins. In one study of fraternal twins, the chance of a female twin giving birth to a set of twins herself was about 1 in 58 births.