Monoamniotic twins (sharing the same amniotic sac): do they do better if born early?

What is the issue?

Identical twins who share the same amniotic sac in the uterus are called monoamniotic twins. The amniotic sac is the bag of waters (amniotic fluid) surrounding the baby. Monoamniotic twins are different from the majority of identical twins, who are separated from each other in their own amniotic sac, with their own amniotic fluid.

Pregnancies with monoamniotic twins are rare – about one in every 10,000 pregnancies is affected, or about 1 per cent of all incidences of identical twins. Monoamniotic twin pregnancies have risks – beyond the risks which apply to all types of twin pregnancy – and these increased risks include the death of both or one of the twins. The higher risks are mostly because the umbilical cords may become compressed, which can be dangerous because these stop the cord from getting oxygen to the baby.

We wanted to find out if it is better to ensure an earlier birth for monoamniotic twins through induction or caesarean section (‘planned early birth’) before 34 weeks pregnancy, or to wait until later, about 36-38 weeks (or until labour starts by itself), keeping a close check on the pregnancy and the twins throughout the pregnancy (‘expectant management’).

Why is this important?

Monoamniotic twin pregnancies have higher risks than other types of twin pregnancy and these increased risks include the death of both or one of the twins. It could be that making sure the twins are born early – before 34 weeks – reduces the risk of cord compression. However, the effects of preterm birth can be serious, too, and the need for hospital care for some time is almost certain.

What evidence did we find?

We looked for randomised controlled trials comparing the outcomes of planned early birth versus expectant management (search date 31 March 2015). We looked for trials in all languages, and with no date restrictions. We did not find any.

There are case series studies and expert clinical opinions, but these types of studies cannot be relied upon to provide a clear answer unless they include very large numbers and are properly assessed for quality.

What does this mean?

Women and their families should discuss with their healthcare providers what the best option is for them. Part of that discussion will include whether high quality neonatal care is available, if early birth is chosen.

Because a randomised controlled trial would be difficult with such a rare problem, we think further research will come from bringing together the results from many different hospitals, along with the social aspects involved.

Authors' conclusions: 

Monoamniotic twins are rare, and there is insufficient randomised controlled evidence on which to draw strong conclusions about the best management. In their absence, we can refer to historical case series and expert consensus. Management plans should take into consideration the availability of high-quality neonatal care if early delivery is chosen. Women and their families should be involved in the decision making about these high-risk pregnancies.

Ongoing, multicentre audits of maternal and perinatal outcomes for monoamniotic twins are needed in order to inform families and clinicians about up-to-date perinatal outcomes with contemporary obstetric practice. Research should consider the social and economic implications of planned interventions, as well as the perinatal outcomes.

Read the full abstract...

Monoamniotic twin pregnancies are formed when a single egg is fertilised and the resulting inner cell mass splits to form twins sharing the same amniotic sac. This condition is rare and affects about one in 10,000 pregnancies overall. Monoamniotic twin pregnancies are susceptible to complications including cord entanglement, increased congenital anomalies, intrauterine growth restriction, twin-to-twin transfusion syndrome and increased perinatal mortality. All twin pregnancies also carry additional maternal risks including pre-eclampsia, anaemia, antepartum haemorrhage, postpartum haemorrhage and operative delivery.

The optimal timing for the delivery of monoamniotic twins is not known. The options include 'planned early delivery' between 32 and 34 weeks, or alternatively awaiting spontaneous labour at least up until the usual time of planned delivery for other monochorionic twins (approximately 36 to 38 weeks' gestation), unless there is a specific indication for earlier delivery.


To assess whether routine early delivery in monoamniotic twin pregnancies improves fetal, neonatal or maternal outcomes compared with 'expectant management'. Expectant management means awaiting spontaneous labour at least up until the usual time of planned delivery for other monochorionic twins (approximately 36 to 38 weeks' gestation in many centres), unless a specific indication for delivery occurs in the meantime, e.g. for non-reassuring antenatal testing.

Search strategy: 

We searched the Cochrane Pregnancy and Childbirth Group's Trials Register (31 March 2015).

Selection criteria: 

Published and unpublished randomised controlled trials (including cluster-randomised trials) comparing outcomes for women and infants who were randomised to planned early delivery of a monoamniotic twin pregnancy with outcomes for women and infants who were randomised to either planned term delivery or expectant management. However, we did not identify any trials for inclusion in this review.

Quasi-randomised controlled trials, trials published in abstract form only, and trials using a cross-over design are not eligible for inclusion in this review.

Data collection and analysis: 

No trials were identified by the search strategy.

Main results: 

No trials were identified by the search strategy.