Although monochorionic monoamniotic (MCMA) twinning is rare, the controversy surrounding the management of MCMA twins continues, thereby making the topic important. Monoamniotic twins are known to have a high perinatal mortality rate, historically reported to be 30–70%. Entanglement of the umbilical cord, leading to compromise of umbilical blood flow, was thought to be responsible for this high mortality. The systematic review published in this issue of the Journal is an attempt to answer important questions relating to MCMA twins by a methodical study of published data.
How common is cord entanglement in monoamniotic twins?
The systematic review by Rossi and Prefumo studied nine published articles and reports that cord entanglement was seen at delivery in 82 out of 148 (55.4%) pregnancies in a subset of four articles that stratified outcomes according to the presence or absence of cord entanglement at birth. Where reported, prenatal ultrasound detected cord entanglement in a large majority (98%) of cases. Rossi and Prefumo assumed that entanglement was not present if it was not documented as such. However, it must be remembered that these studies were not carried out with the primary intention of identifying cord entanglement. Despite the attempts to contact the authors of the original studies, it is possible that cord entanglement was present in higher proportions, but was not identified and/or reported. Indeed, some studies have reported detection of cord entanglement on antenatal ultrasound in every monoamniotic twin pregnancy. Based on this systematic review, it is safe to assume that the minimum prevalence of cord entanglement in MCMA twins is 55.4%.
Is cord entanglement responsible for increased perinatal mortality in monoamniotic twins?
The results of this systematic review show that perinatal morbidity in monoamniotic twins with cord entanglement is actually lower than in those without. This result almost defies logic. However, no significant difference was found in perinatal mortality with or without cord entanglement. We know that the prevalence of congenital abnormalities in MCMA twins is especially high. Traditionally, twin reversed arterial perfusion (TRAP) cases are included in monoamniotic twins. However, it is debatable whether the TRAP mass constitutes a ‘fetus’, and ‘mortality’ of the TRAP mass is inevitable. A perinatal mortality rate of 17% was reported in a large series of MCMA twins, but lethal abnormalities were excluded and the pregnancies had already reached 20 weeks' gestation. The overall survival in this systematic review was 202/228 (89%), making it clear that cord entanglement has minimal impact on perinatal mortality of MCMA twins. It is likely that prematurity, congenital abnormalities and twin-to-twin transfusion syndrome are the more common causes of perinatal mortality in MCMA twins.
How ‘increased’ is perinatal mortality in monoamniotic twins, and how can one reduce it?
The statement that perinatal mortality of monoamniotic twins is increased raises the question: increased compared to what? We have argued that the most appropriate group for comparison is monochorionic diamniotic twins, rather than singletons or dichorionic twins. The many-fold increase in mortality in monochorionic compared with dichorionic twins, particularly before 24–26 weeks, was also shown by Sebire et al. in 1997. In structurally normal monoamniotic twins, perinatal mortality is comparable to that in monochorionic diamniotic twins.
Currently, most centers deliver MCMA twins electively at 32 or 34 weeks. There is a general consensus to deliver monochorionic diamniotic pregnancies electively by 36–37 weeks. This difference in policy for delivery should be questioned if the outcome of monoamniotic twins is confirmed to be comparable to that of monochorionic diamniotic twins in larger series. The next big question is to decide when to deliver structurally normal uncomplicated monoamniotic twins.