Sonographic considerations with multiple gestation.

Determination of chorionicity is of paramount importance in risk assessment and management. Best performed in the first trimester, dichorionic placentation can be reliably assumed when the membrane is easily seen, there is a "twin peak" sign, there are clearly separate placentas, and there is discordant fetal gender. In a monochorionic twin pregnancy, there is a single placental mass, the dividing membrane is difficult to visualize until the end of the first trimester, and the membrane inserts onto the placental surface without a peaked appearance. Amniotic fluid volume assessment is important in the management of twin pregnancy. Polyhydramnios-oligohydramnios may be a manifestation of twin-twin transfusion syndrome, although oligohydramnios with normal amniotic fluid volume in the other twin's sac may more likely be a sign of velamentous cord insertion, infection, or chromosomal or structural abnormality. Fetal growth discordance is common in twin pregnancy and is associated with increased perinatal mortality and morbidity. The most sensitive indicator of discordant twin growth is thought to be estimated fetal weight, and an intertwin difference of > or = 20% is considered significant. In the clinical care of a patient with twins, it is reasonably standard to confirm chorionicity with ultrasonography in the first or early second trimester. At about 20 weeks, a level II ultrasound for anatomic survey is indicated. In dichorionic pregnancies, ultrasound examinations are then performed at 26 to 28 weeks and every 3 to 4 weeks thereafter to follow growth and amniotic fluid volume. In monochorionic twins, we generally do an additional ultrasound at about 23 to 24 weeks, because of the risk of twin-twin transfusion syndrome. In the late third trimester, careful attention should also be given to fetal position, to help with delivery planning.