One proposed management scheme is based on a study by Heyborne and coworkers (2005), who reported no stillbirths in 43 twin pregnancies of women admitted at 26 to 27 weeks' gestation for daily fetal surveillance. However, in 44 women managed as outpatients and admitted only for obstetrical indications, there were 13 stillbirths. Because of this report, women with monoamnionic twins are recommended to undergo 1 hour of daily fetal heart rate monitoring, either as an outpatient or inpatient, beginning at 26 to 28 weeks' gestation. With initial testing, a course of betamethasone is given to promote pulmonary maturation (Chap. 42, p. 823). If fetal testing remains reassuring and no other intervening indications arise, cesarean delivery is performed at 32 to 34 weeks. A second course of betamethasone can be given before this (American College of Obstetricians and Gynecologists, 2016). his management scheme is used at Parkland Hospital and resulted in the successful 34-week delivery of the twins depicted in Figure 45-12.
Of monoamnionic twins just described, one interesting subset derives from embryonic splitting on postfertilization day 9. These "mirror image twins" are genetically identical but have mirror image features such as handedness and hair whorls (Post, 2015).
More seriously, several aberrations in monozygotic twinning result in a spectrum of fetal malformations. hese are traditionally ascribed to incomplete splitting of an embryo into two separate twins. However, it is possible that they may result from early secondary fusion of two separate embryos. These separated embryos are either symmetrical or asymmetrical, and the spectrum of anomalies is shown in Figure 45-13.
In the United States, united or conjoined twins have been referred to as Siamese twins-after Chang and Eng Bunker of Siam (Thailand), who were displayed worldwide by P. T. Barnum. Joining of the twins may begin at either pole and produce characteristic forms depending on which body parts are joined or shared (Fig. 45-14). Of these, thoracopagus is the most common (Mutchinick, 2011). he frequency of conjoined twins is not well established. In Singapore, Tan and coworkers (1971) identified seven cases of conjoined twins among more than 400,000 deliveries-an incidence of 1 in 60,000.
Conjoined twins can frequently be identiied using sonography at midpregnancy (McHugh, 2006). his provides an opportunity for parents to decide whether to continue the pregnancy. As shown in Figure 45-15, identification of cases during the first trimester is also possible. During sonographic interrogation, fetal poles are closely associated and do not change relative position from one another. A targeted examination, including a careful evaluation of the organs involved, is necessary before counseling can be provided. As shown in Figure 45-16, MR imaging is a valuable adjunct to clariY shared organs. Compared with sonography, MR imaging can provide superior views, especially in later pregnancy when amnionic luid is diminished and fetal crowding is greater (Hibbeln, 2012).
Surgical separation of an almost completely joined twin pair may be successful if essential organs are not shared (O'Brien, 2015; Tannuri, 20l3). Conjoined twins may have discordant structural anomalies that further complicate decisions about whether to continue the pregnancy. Consultation with a pediatric surgeon often assists parental decision making. A recent