Monochorionic monoamniotic (MCMA) twins share both a placenta and an amniotic sac. They are a rare type of twins, making up around 1% of all twin pregnancies and 5% of monochorionic twin pregnancies. This occurs when a fertilised egg splits late in its development, leading to only one placenta and amniotic sac for the two babies to share. The chorionicity (number of placentas) and amnionicity (number of amniotic sacs) of the pregnancy should be diagnosed at your dating scan, between 11+2 and 14+1 weeks of pregnancy. If your health professional isn’t sure whether the pregnancy is MCMA and there is still doubt in the diagnosis of chorionicity, you should be referred to a fetal medicine specialist without delay, as chorionicity is best determined before14 weeks of gestation.

MCMA pregnancies are considered high risk, as they are susceptible to complications. Around 20% of MCMA pregnancies will have complications compared to 10% of Monochorionic diamniotic (MCDA) pregnancies, which share a placenta but not a sac. The most common complication in MCMA twins is Twin Reversed Arterial Perfusion sequence (TRAP sequence). Other potential complications include Twin-to-Twin Transfusion Syndrome (TTTS), Twin Anaemia-Polycythaemia Sequence (TAPS) and Selective Fetal Growth Restriction (sFGR or sIUGR), although these are less common in MCMA twins than in MCDA twins. Because the babies are in the same amniotic sac, their umbilical cords can sometimes become tangled and this means there is sadly a risk of sudden and unpredictable death due to cord accidents.

To help manage these risks, MCMA pregnancies are referred to a specialist hospital which has expertise in caring for this type of pregnancy. The pregnancy is scanned every two weeks from 16 weeks onwards, to check for any indicators of complications that may need monitoring or treating. These scans will measure fetal growth and the amount of amniotic fluid, look in detail at the babies’ overall anatomy and development, assess blood flow, any signs of being at risk of preterm birth and the different sizes of the babies in comparison to each other.

If any complications are identified, your health professional will talk you through the treatment options which vary depending on the type of complications and your specific pregnancy. You can read more about the management of these complications in our booklet, Complications in your pregnancy with twins, triplets and more. You will need to register for free to access this.

As MCMA twins have a higher risk of fetal loss a planned caesarean birth between 32+0 and 34+0 weeks of gestation is advised. You should be offered steroids before delivery to help mature the babies’ lungs. If you have any concerns about your pregnancy, speak to your health professional who is best placed to advise you.