There are two ways that a woman may conceive twins. In one case, her ovaries release two eggs at the time of ovulation, and both are fertilized and become embryos; this results in fraternal, or nonidentical, twins. In contrast, identical twins are conceived when one embryo splits into two early in its development.

Does having twins mean you're more fertile?

A history of fraternal twins on the maternal side of the family indicates a higher likelihood of ovulating more than one egg per cycle.

What happens during ovulation or fertilization that causes twins?

Dizygotic twins occur when two sperm fertilize two distinct eggs. In this instance, rather than releasing one egg during ovulation, the mother has released two. (For higher multiple births, more eggs are released - for example, in trizygotic triplets, three eggs are fertilized by three sperm.)