There are two varieties of twin pregnancies. The first occurs when a single egg is fertilized and then splits early in pregnancy, forming two babies. Such twins are known as monozygotic (from one egg), are identical, and are found at a fairly constant rate of one per 1,000 pregnancies, regardless of factors such as a mother's age or race.

The second type of twins arises when two eggs are released, are both fertilized, and both implant in the uterus. Such twins are termed dizygotic (from two eggs) and are not identical (also called fraternal). Rates for dizygotic twinning do vary widely between populations based on factors such as age (older women are at higher risk), race (Asian women are at decreased risk, and African or African-American women are at increased risk compared to Caucasian women), family history, and use of assisted reproductive technologies such as in-vitro fertilization. If a woman has had twins in the past, her chance of having another set is thought to increase by as much as fourfold. If there are twins in her family, the chance of her having twins is also increased: The exact chance will be determined by the combination of all maternal characteristics, but in almost all cases will be less than 5 percent. (History of twins in the father's family is thought to have no effect).

In this country the number of twin pregnancies has been rising, largely as a result of advancing average age of mothers at delivery and, more importantly, increasing use of fertility medications and procedures. These techniques often include stimulating the ovaries to produce many eggs. When medications such a Clomid are used to cause such super-ovulation, many eggs can be fertilized. With in-vitro fertilization, many eggs are collected and two or more embryos may be returned for possible implantation in the uterus. Prudent fertility specialists work very hard to balance the chances of successful pregnancy (increased by returning more embryos or causing more eggs to be ovulated) against the risks associated with twin and higher-order multiple gestations (triplets, quadruplets and beyond).