Multiple gestations (twins, triplets and higher order) represent approximately 3.4% of all births in the United States. Compared to singletons, these pregnancies are at a higher risk of preterm delivery and low birth weight. In addition, they may develop specific complications depending on whether the fetuses share a common placenta (monochorionic) or not (dichorionic, trichorionic) and on the number of fetal sacs.

Because some of the complications of twins and triplets are associated with whether the fetuses share a common placenta or not, the diagnosis of the type of twin pregnancy (monochorionic -one shared placenta, dichorionic – separate placentas, not shared) is essential. In addition, the diagnosis of the number of fetal sacs is also very important, particularly to rule out the presence of two or more fetuses within the same sac.

The diagnosis of chorionicity (number of placentas) and amnionicity (number of sacs) is best made at the beginning of the pregnancy, for example, at the time of the 11-13 6/7 week ultrasound. Thus, patients with known multiple pregnancies are advised to have an ultrasound at this gestational age and to have the diagnosis of chorionicity and amnionicity made at that time. A generic diagnosis of “twins” or “triplets” is considered insufficient.

Specific complications in monochorionic twins include a higher rate of birth defects, twin-twin transfusion syndrome (TTTS), selective intrauterine growth restriction (SIUGR) and twin-anemia polycythemia syndrome (TAPS). Monoamniotic twins (2 fetuses in a single sac) can have a higher rate of fetal death and other problems. Complications seen in singletons, like fetal anemia, also represent an additional management challenge in monochorionic twins.

Monochorionic twins have a 3-4 times higher rate of birth defects compared to singletons. In contrast, dichorionic twins (twins with separate placentas) have a similar rate of birth defects than singletons. Birth defects in monochorionic twins typically affect only one of the twins and are thus called discordant. The presence of a birth defect in one monochorionic twin may or may not affect the prognosis for the co-twin, depending on the type of anomaly.

Twin-twin transfusion syndrome (TTTS) is a complication of monochorionic twins in which an uneven exchange of blood between the fetuses through placental vascular anastomoses produces oligohydramnios in the donor twin and polyhydramnios in the recipient twin. Untreated, TTTS results in the loss of most pregnancies or in very early preterm birth. Definitive treatment of TTTS is achieved by occluding with laser energy the placental vascular anastomoses responsible for the syndrome.

Selective intrauterine growth restriction (SIUGR) occurs when the estimated fetal weight of one of the fetuses is below the 10th percentile. Management of SIUGR depends on additional complicating factors, such as blood flow studies. Although most pregnancies with SIUGR can be managed expectantly, laser therapy, similar to the one performed for TTTS, may be needed in a small subset of patients.

Twin-anemia-polycythemia syndrome (TAPS) occurs when one of the twins is suspected as being anemic, while the other twin has an excess of red blood cells (polycythemic). The cause of this complication of monochorionic twins is unknown. Management may involve a combination of strategies, including laser therapy similar to the one performed for TTTS, intrauterine transfusion of the anemic twin, or a combination of these.

Monoamniotic twins (2 fetuses within the same sac), aside from being at risk for all of the complications seen in the other types of monochorionic twins, are at risk of becoming entangled with their umbilical cords. The resulting cord entanglement may jeopardize the life of one or both twins. Management of these pregnancies is extremely special, and may require a combination of medical treatments to limit the excursion of movements of the fetuses within the sac, or surgical intervention.