Ultrasound evaluation of twin pregnancies.

Twin pregnancies are at increased risk of perinatal morbidity and mortality. Ultrasound imaging is a critical component of twin management, allowing earlier identification and management of potential complications. Ultrasound for twin gestations should begin in the first trimester, and the timing of follow-up scans is based on the chorionicity and amnionicity of the gestation. In dichorionic twins, ultrasound can detect discordant malformations, discordant growth, and increased risk for preterm delivery. In monochorionic twins, ultrasound not only allows similar surveillance as for dichorionic twins but also monitors for evidence of twin-twin transfusion syndrome (TTTS), twin anemia polycythemia sequence (TAPS), selective fetal growth restriction (sFGR), and twin reversed arterial perfusion (TRAP) sequence that may require in utero surgical intervention.