Sonographic features used to evaluate chorionicity vary according to gestational age. Accuracy is greatest in the irst trimester and diminishes as gestational age advances. Namely, chorionicity can be determined sonographically with 98-percent accuracy in the irst trimester but may be incorrect in up to 10 percent of second-trimester examinations (Emery, 2015; Lee, 2006). Moreover, for sonographic evaluations between 15 and 20 weeks' gestation, the odds of chorionicity misclassiication rise by approximately 10 percent for each week of advancing gestational age in pregnancies compared with those completed before 14 weeks (Blumenfeld, 2014). Overall, chorionicity can be correctly determined with sonography before 24 weeks in approximately 95 percent of cases (Lee, 2006).
Early in the first trimester, the number of chorions equates to the number of gestational sacs. A thick band of chorion separating two gestational sacs signals a dichorionic pregnancy, whereas monochorionic twins have a single gestational sac. If the gestation is monochorionic diamnionic, it may be diicult to visualize the thin intervening amnion before 8 weeks' gestation (Emery, 2015). If the intervening membrane is diicult to visualize, the number of yolk sacs usualy correlates with the number of amnions. However, the number of yolk sacs as a predictor of amnionicity may not always be accurate (Shen, 2006). Although uncommonly seen early, cord entanglement identifies a monoamnionic gestation. When chorionicity is uncertain, additional later sonographic examinations are performed.
Ater 10 to 14 weeks' gestation, sonographic assessment of chorionicity may be determined using four features. These are the number of placental masses, thickness of the membrane dividing the sacs, presence of an intervening membrane, and fetal gender (Emery, 2015). First, two separate placentas suggest dichorionicity. The converse is not necessarily true, such as cases with a single fused placental mass. Second, identiication of a thick dividing membrane-generally �2 mm-supports a presumed diagnosis of dichorionicity. In a dichorionic pregnancy, this visualized membrane is composed of a total of four layers-two amnion and two chorion. Also, the twin peak sign-also called lambda or delta sign-is seen by examining the point of origin of the dividing membrane on the placental surface. he peak appears as a triangular projection of placental tissue extending a short distance between the layers of the dividing membrane (Fig. 45-4).
FIGURE 45-4 A. Sonographic image of the "twin-peak" sign, also termed the "lambda sign," in a 24-week gestation. At the top of this sonogram, tissue from the anterior placenta is seen extending downward between the amnion layers. This sign confirms dichorionic twinning. B. The "tWin-peak" sign is seen at the bottom of this schematic diagram. The triangular portion of placenta insinuates between the amniochorion layers.
FIGURE 45-5 A. Sonographic image of the '" sign in a monochorionic diamnionic gestation at 30 weeks. B. Schematic diagram of the '" sign. Twins are separated only by a membrane created by the juxtaposed amnion of each twin. A '" is formed at the point at which amnions meet the placenta.