Multiple pregnancy is the growth of more than one baby during a woman’s pregnancy. Most multiple pregnancies involve twins, but births of triplets and quadruplets have become more common.

Pregnancy occurs when a woman’s egg is united with a man’s sperm and this fertilized egg implants and grows in the uterus. A multiple pregnancy occurs when more than one egg is available to be fertilized (non-identical) or when the fertilized egg divides into identical but separate fertilized eggs (identical) very early in the process. Multiple pregnancies occur naturally and frequently occur among women who use fertility drugs and assisted reproductive technology (ART). Fertility drugs can increase the number of eggs released at ovulation, and ART methods (e.g., in-vitro fertilization) require the implanting of multiple embryos. This is performed to help improve the rate of a successful pregnancy because not all of the embryos may develop.

For pregnant women, there are many health considerations that are further heightened when they are carrying more than one baby. These women have increased nutritional needs and gain more weight than during a single pregnancy. Women with a multiple pregnancy are more likely to miscarry and more likely to face other complications such as gestational diabetes, preeclampsia and preterm labor. Multiple pregnancies will require more visits to the obstetrician-gynecologist (ObGyn) and some women choose to use an ObGyn who specializes in high-risk pregnancies (maternal-fetal specialist).

The most common complication of a multiple pregnancy is premature birth. More than half of twin deliveries occur prematurely. The likelihood of an early delivery increases with each additional baby in the pregnancy. Infants born prematurely have lower birth weights and are susceptible to other complications such as cerebral palsy, mental retardation and respiratory distress syndrome.