Multiple pregnancy is a pregnancy with more than one fetus. The most common type of multiple pregnancy is twins, followed by triplets and quadruplets. Multiple pregnancies can occur naturally, or they can be the result of assisted reproductive technology (ART), such as in vitro fertilization (IVF).

How does multiple pregnancy happen?

There are two main ways that multiple pregnancy can happen:

• Dizygotic twins: Dizygotic twins are the result of two separate eggs being fertilized by two separate sperm. This can happen naturally, or it can be the result of ART.
• Monozygotic twins: Monozygotic twins are the result of a single egg splitting into two embryos. This can happen at any point during the early development of the pregnancy.

What are the risks and complications of multiple pregnancy?

Multiple pregnancies are at an increased risk of a number of complications, including:

• Preterm birth: Preterm birth is the birth of a baby before 37 weeks gestation. Multiple pregnancies are more likely to deliver preterm than singleton pregnancies.
• Low birth weight: Low birth weight is the birth of a baby who weighs less than 5 pounds, 8 ounces. Multiple pregnancies are more likely to have low birth weight babies than singleton pregnancies.
• Preeclampsia: Preeclampsia is a pregnancy complication characterized by high blood pressure and protein in the urine. Multiple pregnancies are more likely to develop preeclampsia than singleton pregnancies.
• Twin-to-twin transfusion syndrome (TTTS): TTTS is a rare complication of multiple pregnancy that occurs when twins share the same placenta and there is an imbalance in blood flow between the twins.