Yolk sac number does not predict reliably amnionicity in monochorionic twin pregnancies: a case of a monochorionic monoamniotic twin pregnancy with two distinct yolk sacs on early first‐trimester ultrasound

The dramatic increase in multiple births over the past 30 years has made multiple pregnancy one of the most common high-risk conditions encountered by obstetricians. In North America, the rate of multiple pregnancy has increased by 50–70% and this is largely attributed to advanced reproductive technologies1, 2. In monochorionic multiple gestations the determination of amnionicity is technically difficult to perform based both on the relatively late appearance of the amnion on ultrasound, typically between 8 and 10 weeks, along with the thin nature of the membrane. Previous studies have suggested that on early first-trimester ultrasound, monochorionic monoamniotic (MCMA) twin pregnancies can be reliably characterized by the presence of a single yolk sac and monochorionic diamniotic (MCDA) twins can be reliably characterized by the identification of two yolk sacs3. Accurate and early prenatal ultrasound diagnosis of amnionicity, along with enhanced prenatal/neonatal care, has been shown to decrease the mortality rates of MCMA twins from 50% to 10–20% in more recent years2. Recently, the accuracy of using yolk sac number as a means of predicting amnionicity has been challenged4-6. In a recent series of 22 cases of monochorionic multiple gestations, Shen et al.4 revealed that 85% of the 20 cases of MCDA gestations had two yolk sacs and that 15% were characterized by only a single yolk sac; a single case of MCMA was identified in this series and one yolk sac was seen. In 2010, three cases of MCMA twins characterized by the presence of two yolk sacs on first-trimester ultrasound were reported5, 6. Here, we present a fourth case of MCMA twins in which early first-trimester ultrasound demonstrated the presence of two yolks sacs with confirmed monoamnionicity in the late first trimester. A 31-year-old G1P0 Caucasian woman presented at a private fertility clinic for intrauterine insemination with donor sperm for social reasons. A transvaginal ultrasound examination (E8, GE Medical Systems, Zipf, Austria) was performed at 7 + 0 weeks for confirmation of intrauterine pregnancy. The 7-week ultrasound clearly demonstrated two distinct fetal poles and two separate yolk sacs. The crown–rump length measurements, of 7.6 mm and 7.9 mm, were consistent with the expected size for each fetal pole, and cardiac activity was present in both. The pregnancy was considered to be MCDA twin gestation at this point in time, based on the presence of two yolk sacs (Figure 1). At 9 + 2 weeks a single amniotic sac was observed with the absence of a dividing intergestational membrane (Figure 2). At 12 + 2 weeks of gestation, ultrasound demonstrated the absence of a dividing membrane between the fetuses (Figure 3). Power Doppler at this time revealed umbilical cord entanglement, confirming the diagnosis of a MCMA twin gestation (Figure 4). Transvaginal ultrasound at 7 weeks' gestation demonstrated two yolk sacs in a monochorionic twin pregnancy. Repeat transvaginal ultrasound at 9 + 2 weeks revealed a single amniotic sac and the absence of an intergestational membrane. Transvaginal ultrasound at 12 + 2 weeks demonstrated the absence of a dividing membrane. Transvaginal ultrasound at 12 + 2 weeks verified umbilical cord entanglement. This case highlights the close proximity in the sequence of development of the yolk sac and amnion and emphasizes our lack of understanding of the temporal differentiation of these structures. The almost simultaneous development of the yolk sac and amnion could explain these most recent findings that yolk sac number does not always correlate with amnionicity in monochorionic gestations. Yolk sac number should no longer be considered a reliable means of diagnosing amnionicity on early first-trimester ultrasound. The consequent clinical implications are an enhanced focus on the diagnosis of amnionicity based on the presence or absence of the intergestational amnion and the visualization of umbilical cord entanglement7, 8.

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