Most pregnancies are the product of a single fertilized ovum and result in a single fetus. One to two percent of pregnancies, however, result in multiple fetuses, multigestational pregnancy. Such pregnancies present unique challenges for both prenatal management and delivery. Although uncommon, multigestational pregnancy occurs with sufficient frequency that primary care providers should be familiar with basic management considerations.

Multiple gestations may be the result of either a single fertilized ovum that divides early in development or multiple fertilized ova from the same cycle. Monozygotic (MZ) twins are genetically identical fetuses produced from a single fertilized ovum. MZ twins make up about 30 % of all twin pregnancies. Dizygotic (DZ) twins are the product of two separate fertilized ova. Although they are genetically similar, they are not genetically identical. DZ twins make up 70 % of all twin pregnancies with a frequency of approximately 1 out of 80 pregnancies. Although significantly less common, multigestational pregnancy may result in more than two developing fetuses. The expected natural frequency of multigestational pregnancy is approximately 1 in 80 twins, 1 in 6400 triplets, and 1 in 512,000 quadruplets. The increase in assisted fertility has significantly altered the frequency of multigestational pregnancies, however, and such figures may no longer be entirely applicable.

In addition to multiple fetuses, multigestational pregnancies may have several variations of multiple chorions and placentae. MZ twins may have a single placenta and a single chorion (~60 %) or may have two chorions with either a fused placenta or two separate placentae (~20 % each). DZ twins have two chorions and two placentae, which may be fused or separate (~50 % each).

Multigestational pregnancies are associated with increased risk for a variety of prenatal and delivery-related complications (see Table 16.1). Maternal risks associated with multigestational pregnancy include spontaneous abortion, stillbirth, preterm labor, preterm delivery, placental previa, anemia, urinary tract infection (UTI), pre-eclampsia, and postpartum hemorrhage. Up to two-thirds of twin pregnancies will result in loss of one twin in the first trimester. The rate of fetal demise is twice as high as for single pregnancies. The risk of both anemia and UTI is 2–3 times normal. There is a threefold increased risk of pre-eclampsia and a fivefold increased risk of postpartum hemorrhage in multigestational pregnancy.