The chance of a pregnant woman giving birth to twins is governed by multiple genetic and environmental factors. Monozygotic or identical twins are born when a single egg cell is fertilized by a single sperm cell, giving rise to two separate embryos. Research shows that the incidence of identical twins is not due to genetic factors.

On the other hand, dizygotic or fraternal twins are born when two egg cells are fertilized by two different sperm cells during the same menstrual cycle. Fraternal twins are more common than identical twins. The incidence of such twins happens to run in families. For example, if a close relative (sister) has fraternal twins, it is likely that one will have twins too.