Monoamniotic twinning is an unusual form of twinning in which both twins occupy a single amniotic sac. Monoamniotic twins account for 1% of all monozygotic twin pregnancies (Benirschke and Kim, 1973). This form of monozygotic twinning typically occurs when a single embryo splits between the ninth and twelfth day after fertilization. Splitting before this time gives rise to either dichorionic diamniotic twins (split between the first and third day after fertilization) or to monochorionic diamniotic twins (split between the third and eighth day after fertilization). Splitting later in embryogenesis (after the twelfth day postfertilization) gives rise to conjoined twins (Benirschke, 1998).

Although a rare event, monoamniotic twinning is important because of the high perinatal mortality rate associated with these pregnancies. The first comprehensive review of the world literature was performed in 1935 by Quigley, who found an overall mortality rate of 68% in 94 pregnancies. His opinion was that the poor prognosis was due mainly to twisting and knotting of the umbilical cords with subsequent occlusion of the blood supply to one or both twins. A subsequent review in 1959 added 35 new cases to the world literature and reported a high fetal mortality rate of 30% (Salerno, 1959). More recent series and reviews of prenatally diagnosed cases suggest mortality rates ranging from 10% to 32% (Rodis et al., 1997; Allen et al., 2001; Roque et al., 2003; Demaria et al., 2004; Heyborne et al., 2005). This decrease in perinatal mortality is likely secondary to increased rates of prenatal diagnosis, antenatal steroids, intense fetal surveillance, and timed delivery.

It is difficult to ascertain the exact incidence of monoamniotic twins. The incidence has ranged in various studies from 1 in 1650 to 1 in 93,734 livebirths (Simonsen, 1966; Colburn and Pasquale, 1982). Monoamniotic triplets are even more rare (Giannopoulos et al., 2001). Use of in vitro fertilization and embryo transfer may increase the risk for both monochorionic diamniotic twins and for monoamniotic twins (Alikani et al., 2003).

There may be a preponderance offemale twins among monoamniotic twin pairs (Derom and Vlietinck, 1988). In a study of 26 sets of monoamniotic twins, 20 (77%) were female pairs. However, this difference has not been as evident in more recent studies (Carr et al., 1990; Allen et al., 2001). Carr et al.’s (1990) series of 24 sets included 11 (46%) sets of females and 13 (54%) sets of males. Allen et al.’s (2001) series of 25 sets included 12 (48%) sets of females and 13 (52%) sets of males.

Prenatal diagnosis is established when a dividing membrane cannot be identified by an experienced sonographer in a twin gestation with a single placental mass and like-gender twins. The diagnosis is confirmed after sonographic identification of entangled umbilical cords using Color flow Doppler (Belfort et al., 1993; Aisenbrey et al., 1995) (Figure 122-1). This feature has been reported in 70% to 100% of cases (Lee, 1992; Rodis et al., 1997). Cord entanglement has been diagnosed using Color Doppler ultrasonography as early as 10 weeks’ gestation (Arabin et al., 1999; Sebire et al., 2000; Sherer et al., 2002).

Monoamnionicity may be suspected early in the first trimester. Between the sixth and tenth week, counting the number of gestational sacs is an accurate method of determining chorionicity. Amnionicity can be determined by counting the number of embryonic heartbeats in each gestational sac. A single gestational sac with two embryonic heartbeats can be either monochorionic diamniotic or monochorionic monoamniotic. Two yolk sacs indicate that the pregnancy is diamniotic. One gestational sac with one yolk sac and two embryonic heartbeats suggests monoamnionicity. If monoamnionicity is suspected in the first trimester, a follow-up ultrasound is suggested in the second trimester to reconfirm the diagnosis (Bromly and Benacerraf, 1995). Three-dimensional ultrasound has also been used to visualize monoamniotic fetuses lying in a single amniotic cavity (Su, 2002).

It is important to remember that failure to visualize a dividing membrane does not ensure that the pregnancy is monoamniotic (Malone and D’Alton, 2000). Such a membrane may be missed even with well-performed sonography (Blane et al., 1987). As a result, other techniques have been utilized to confirm the diagnosis of monoamnionicity. Ultrasound-guided injection of indigo carmine dye mixed with air into the amniotic sac during genetic amniocentesis has been said to enhance the diagnosis of monoamniotic twins (Tabsh, 1990). If microbubbles are seen around both fetuses, the diagnosis of monoamniotic twinning is made with accuracy. Amniography is another technique, which may increase the accuracy of diagnosis (Lavery and Gadwood, 1990). In this report, 30 mL of 61% iopamidol (Isovue-M 300, Squibb) was injected into the amniotic fluid. Maternal abdominal x-ray films taken 24 hours after the injection revealed that both fetuses had swallowed contrast medium injected into a shared single amniotic sac. Computed tomography following intraamniotic injection of Renografin has also been suggested to confirm the diagnosis of monoamniotic twinning (Carlan et al., 1990; Perkins and Terry, 1992). Such invasive radiological procedures are now mostly of academic interest, and are rarely if ever performed in contemporary clinical practice.

A major consideration in differential diagnosis of monoamnionicity is the intrauterine rupture of a diamniotic twin membrane, which can mimic the sonographic appearance of true monoamniotic twins. Actual differential diagnosis between these two conditions is probably not necessary, as intrauterine membrane rupture has a perinatal mortality rate consistent with that of true monoamniotic gestations (Gilbert et al., 1991). This clinical scenario should be suspected when there is failure to visualize a dividing membrane following previous sonographic confirmation of a membrane. Possible causes for intrauterine rupture include trauma during amniocentesis, infection, and developmental disturbances of the membranes (Gilbert et al., 1991).

High intrauterine morbidity and mortalityrates have been associated with monoamniotic twins (Quigley, 1935; Salerno, 1959; Raphael, 1961; Simonsen, 1966; Carr et al., 1990; D’Alton and Simpson, 1995). These high rates have been attributed to premature delivery, growth restriction, congenital anomalies, vascular anastomoses between twins, and umbilical cord entanglement and cord accidents. Discordant anatomical abnormalities are common. In a recent series of 25 monoamniotic twin pairs by Allen et al., 28% of the pregnancies had been complicated by fetal structural anatomical abnormalities including two cases of a cardiac twinning (Allen at al., 2001). The twin–twin transfusion syndrome (TTTS) is thought to be rare in monoamniotic twins as the placentas of monoamniotic twins have significantly greater numbers of both superficial and deep anastomoses than uncomplicated monochorionic twins which may be protective against TTTS (Bajoria, 1998; Umur et al., 2003). Cord accidents seem to be the greatest contributor to these high intrauterine mortality rates. In one of the earliest series of monoamniotic twins it was stated that double survival of such twins is a rare event (Quigley, 1935). However, more recent series and reviews of prenatally diagnosed cases suggest mortality rates range from 10% to 32% (Rodis et al., 1997; Allen et al., 2001; Demaria et al., 2004).