In the past, the mortality rate among the pump twins exceeded 50 percent. This stemmed largely from complications of prematurity or from a prolonged high-output state leading to cardiac failure (Dashe, 2001). Risk appears to be directly related to size of the acardiac twin. One sonographic method to estimate acardiac twin size uses the volume of an ellipse: length X width X height x \/6. When the acardiac twin volume is < 50 percent of that of the pump twin, expectant management may be reasonable given the inherent risks of fetal intervention (Chap. 15, p. 326)(Jelin, 2010). When the volume of the
FIGURE 45-23 Photograph of an acardiac twin weighing 475 grams. The underdeveloped head is indicated by the black arrow, and its details are shown in the inset. A yellow clamp is seen on its umbilical cord. Its viable donor cotwin was delivered vaginally at 36 weeks and weighed 2325 grams. (Used with permission from Dr. Michael D. Hnat.) acardiac twin is large, however, treatment has generally been ofered. Radiofrequencyrablation (RFA) is the preferred modality of therapy, and contemporary reports now suggest improved perinatal outcomes. he North American Fetal Therapy Network reviewed their experiences with 98 cases from 1998 to 2008 in which RF A of the umbilical cord was performed (Lee, 2013). Median gestational age at delivery was 37 weeks, and 80 percent of neonates survived (Lee, 20l3). he average gestational age at the time of the RF A was 20 weeks, and the estimated acardius-to-pump twin volume on average was 90 percent. Major complications were prematurely ruptured membranes and preterm birth.
Interestingly, TRAP sequences can also occur within monoamnionic pregnancies. he perinatal outcomes of such pregnancies appear to be worse than that of monochorionic diamnionic cases. Sugibayashi and associates (2016) in a review of 40 cases recently reported that pump twin survival following RF A was 88 percent in monochorionic diamnionic pregnancies but only 67 percent in monoamnionic pregnancies.
• Hydatidiform Mole with Coexisting Normal Fetus his unique gestation contains one normal fetus, and its cotwin is a complete molar pregnancy. Reported prevalence rates range from 1 in 22,000 to 1 in 100,000 pregnancies (Dolapcioglu, 2009). It must be diferentiated from a partial molar pregnancy, in which an anomalous singleton fetususually triploid-is accompanied by molar tissue (Fig. 20-4, p. 391). At times, a twin pregnancy may occur with a normal twin in one sac and a partial mole in the other sac (McN amara, 2016).
Diagnosis is usually made in the first half of pregnancy.