What is a multiple pregnancy?

A pregnancy of two or more fetuses is called a multiple pregnancy. Multiple fetuses can be the same (identical) or different (fraternal).

See a picture of identical and fraternal twins in the womb (uterus).

Identical twins or triplets come from a single egg that has been fertilized by one sperm. For unknown reasons, the fertilized egg splits into two or more embryos during the first stage of development. Some identical twins share the same placenta. But they usually grow within separate amniotic sacs in the uterus. In rare cases, identical twins share one amniotic sac.

Identical twins:

• Are always the same sex and blood type.
• Usually share the same physical features, such as skin, hair and eye color, and body build. But they do not always look exactly alike. They do not have the same fingerprints. Also, one may be right-handed while the other is left-handed.
• Are thought to develop at random. They are not related to your age, race, or family history of twins.¹

Fraternal twins, triplets, or more develop when two or more eggs are fertilized by separate sperm. Fraternal fetuses have separate placentas and amniotic sacs.

Fraternal multiples:

• Can be either gender (male or female) and can have different blood types.
• May look very different from one another, with different-colored hair and eyes. They may also look alike, as siblings often do.
• Tend to run in families.

Multiples that are conceived with infertility treatment are almost always fraternal, rather than identical.

What are common causes of multiple pregnancy?

Infertility treatment. Since 1980, infertility treatment has more than doubled the U.S. twin birth rate. The rate of triplet-or-more births is now nearly 5 times higher than before infertility treatment was used.²

You are more likely to have a multiple pregnancy if you use fertility drugs or assisted reproductive technology (ART).

• Fertility drugs stimulate your ovaries to release more than one egg, which can be fertilized at the same time. This may result in a multiple pregnancy.
• ART is the transfer of multiple embryos directly into the uterus to increase your chances of pregnancy.³ Overall, 25% to 30% of pregnancies from ART are twins; 5% are triplets; less than 1% are quadruplets or more.¹

Other causes. Certain factors increase your risk for having a multiple pregnancy.

• Age older than 35.²
• Race. Fraternal twins are most common among women of African descent, especially between ages 35 and 45.⁴ They are least common among Asian women.⁴
• Personal history. If you have already carried fraternal twins, you have double the usual risk of having twins.¹
• Family history. If your family has a history of fraternal twins or triplets, you have an increased risk of multiple pregnancy. (A history on the father's side of the family does not increase your risk of multiple pregnancy.)⁴
• Timing. You are more likely to conceive multiples soon after stopping birth control pills.⁴

What are the risks of multiple pregnancy?