When diamniotic twins suddenly become monoamniotic twins: spontaneous septostomy of the dividing membrane

Discussion

One potential but very uncommon complication of MCDA pregnancies is SSDM, which results in a pseudomonoamniotic pregnancy with possible cord entanglement, thus increasing the rate of fetal complications and changing prenatal and perinatal management.

The incidence of spontaneous rupture of the intertwin-dividing membrane is unknown. The scarcity of reports on SSDM in the literature suggests that this is not a common phenomenon. The aetiology of SSDM includes fetal trauma, infection and developmental disorder. In the current literature, the incidence of spontaneous septostomy in uncomplicated MCDA twin pregnancies is unknown, but 1.8% has been reported in a high-risk unit.4

Multiple aetiologies of SSDM have been described. The most common is traumatic disruption of the membrane that occurs due to invasive procedures such as amniocentesis, cordocentesis, amnioreduction and laser ablation. Maternal abdominal trauma with concurrent SSDM has also been reported. Other possible causes due to abnormal membrane development have been suggested as chorioamnionitis, fetal movement, amniotic plica and increase of amniotic fluid pressure with polyhydramnios.4 7 8

A retrospective study by Chmait in 2009 with 217 complicated MCDA pregnancies with no prior invasive procedures but with suspicion of TTTS, sIUGR and TRAP sequence or discordant anomalous twins, reported an incidence of SSDM in 1.8% (four cases). This high incidence might be explained by the selected risk patients in this group and TTTS and TRAP with polyhydramnios as a potential risk factor for SSDM.4 9 10

According to Chmait, the most useful sonographic findings for the SSDM diagnosis are: absence or interruption of the dividing membrane; both fetuses occupying the same side of the dividing membrane; umbilical cord entanglement and excess amniotic fluid on both sides of the dividing membrane, with characteristics suggestive of TTTS. The diagnosis of TTTS might be challenging here due to the absence of oligohydramnios.2 3 6

In our case, only the first and second of these ultrasound characteristics were present. We observed no entanglement either on antenatal ultrasound or postnatally.

A review of the literature, revealed 38 cases of SSDM, and associated prenatal complications included fetal and neonatal death secondary to cord entanglement, premature rupture of membranes, premature labour and amniotic band syndrome. This stresses the importance of exclusion of associated anomalies including amputation, constriction bands, syndactyly, craniofacial defects, clubfeet and cleft lip.4 5 7 8 11–13 The neonates’ perinatal mortality rate was as high as 44%, and the mean gestational age at birth was 29 weeks (range 22–34 weeks).7

Prompt recognition of SSDM in MCDA twin pregnancies is important because it can help to establish follow-up guidelines to detect the previously mentioned potential complications, especially umbilical cord entanglement,2 a condition reported by López Ramón and Cajal in a series with five cases of true umbilical cord knot.9 The authors observed a cross-sectional image of the umbilical cord with its three vessels, surrounded by a complete loop; a condition termed the ‘hanging noose’ sign, which represents a characteristic ultrasound image.

Data from a 2019 systematic review and meta-analysis showed that fetal loss occurs in 1% of MCMA twins at 31–32 weeks and 2% at 33–34 weeks of pregnancy14 Double fetal death occurs in approximately 2% of MCMA twins at 33–34 weeks of pregnancy.14 Currently, elective delivery at 32 weeks appears to be the most appropriate decision; however, the risk of neonatal morbidity needs to be considered.

The Royal College of Obstetricians and Gynaecologists issued guidelines in 2016 for managing monochorionic twin pregnancies,14 which clarified that MCMA twin pregnancies have a high risk of fetal death and should be delivered by caesarean section between 32 0/7 and 34 0/7 weeks. Although the condition is fairly rare, SSDM should be considered for an MCDA that shows any of the indicators for SSDM given above—even with no history of invasive surgery, preterm premature rupture of the membranes or invasive prenatal diagnosis, which clarified that MCMA twin pregnancies have a high risk of fetal death and should be delivered by caesarean section between 32 0/7 and 34 0/7 weeks. Although the condition is fairly rare, SSDM should be considered for an MCDA, even with no history of invasive surgery, preterm premature rupture of the membrane, or invasive prenatal diagnosis that shows any of the indicators for SSDM given above.

The principle of gestational management of pseudoamniotic twin pregnancy is to minimise the risk to the fetuses. Possible complications such as umbilical cord entanglement and particularly high perinatal morbidity demand antepartum management similar to that of a functional monoamniotic gestational pregnancy. Early diagnosis of possible complications is very essential, so weekly ultrasound assessment for hydrops, TTTS, assessment of fetal growth is recommended. Depending on the various risk factors and the gestational age, ideally 32–34 weeks, the decision to delivery should be considered.

Consequently, in an MCDA pregnancy, the recommendation should be to frequently evaluate the intertwin membrane by ultrasound. Suppose the dividing membrane could not be observed in ultrasound. One should consider the possibility of spontaneous antepartum septostomy, which would require meticulous observation of the pregnancy entailing a different planning of delivery.11 14

In conclusion, the diagnosis of SSDM poses diagnostic and therapeutic challenges as it might easily be overlooked but is possible and helpful to avoid complications of complications typical for MCMA pregnancies in MCDA twins.