MCMA Monoamniotic Twins

What MCMA means

NICE NG137 defines monochorionic monoamniotic twins as babies who share a placenta and share an amniotic sac. There is no membrane between them. The NHS calls this a much rarer type of pregnancy and notes that there is often some cord entanglement, which can cause complications. Tommy's puts the distribution across twin pairs at roughly 70 to 75 in 100 dichorionic diamniotic, 20 to 30 in 100 monochorionic diamniotic, and 1 or 2 pairs in 100 monochorionic monoamniotic.

Every figure in this article applies to monoamniotic twins specifically. The gestations and intervals for dichorionic or monochorionic diamniotic twins are different, and using the wrong set is the single most common error in twin information.

How it is diagnosed

NG137 says chorionicity and amnionicity should be determined at the time the twin pregnancy is detected, using the number of placental masses, the presence of amniotic membranes and membrane thickness, and the lambda or T-sign. A monoamniotic pregnancy is one where no dividing membrane can be found. NG137 also says that if chorionicity or amnionicity cannot be established, a second opinion should be sought from a senior sonographer, and that a transvaginal scan should be used if transabdominal views are poor.

Where your care happens

NG137 does not leave this to local arrangement. It says women with a twin or triplet pregnancy involving a shared amnion should be offered individualised care from a consultant in a tertiary level fetal medicine centre. It repeats the point in its list of referral indications, where pregnancies with a shared amnion, including monochorionic monoamniotic twins, are named first.

A tertiary level fetal medicine centre is defined in NG137 as a specialist regional or supra-regional centre with the expertise and infrastructure to assess and manage complicated twin and triplet pregnancies, including complex fetal interventions. It may not be your local hospital.

Cord entanglement

Because the babies share a sac, their umbilical cords occupy the same space and can become entangled. The NHS says there is often some cord entanglement in MCMA pregnancies and that this can cause complications. It is the defining risk of this type of pregnancy and the reason the monitoring and the birth timing are what they are.

Monitoring

MCMA twins are monochorionic, so the monochorionic monitoring schedule applies. NG137 says women with a monochorionic twin or triplet pregnancy should be offered diagnostic monitoring for feto-fetal transfusion syndrome with ultrasound every 14 days from 16 weeks until birth, and should be offered monitoring for fetal weight discordance at every ultrasound scan from 16 weeks at intervals not exceeding 14 days. RCOG's patient information puts it as scans at least every two weeks from 16 weeks for MCDA or MCMA twins.

NG137 also asks for simultaneous monitoring for feto-fetal transfusion syndrome, fetal growth restriction and advanced-stage twin anaemia polycythaemia sequence at every ultrasound assessment, explaining that the relative likelihood of each changes with gestation but that they can all occur at any gestational age. Beyond that, care is individualised by the tertiary centre, which is exactly what NG137 asks for.

When MCMA twins are born

NG137 gives a specific window and a specific reason. It says women with an uncomplicated monochorionic monoamniotic twin pregnancy should be told that planned birth between 32+0 and 33+6 weeks does not appear to be associated with an increased risk of serious neonatal adverse outcomes, and that continuing the pregnancy beyond 33+6 weeks increases the risk of fetal death.

It then says to offer planned birth between 32+0 and 33+6 weeks, after a course of antenatal corticosteroids has been considered. The NHS states the same window as 32 to 33 weeks for MCMA twins in its list of planned birth timings.

Note the distance from the other types. NG137 offers planned birth at 37 weeks for uncomplicated dichorionic diamniotic twins and at 36 weeks for uncomplicated monochorionic diamniotic twins. Monoamniotic twins are offered birth around a month earlier than the monochorionic diamniotic group.

Mode of birth is a recommendation, not a choice

For dichorionic and monochorionic diamniotic twins, NG137 says planned vaginal birth and planned caesarean are both safe choices if four conditions are met. Monoamniotic twins are handled separately. NG137 says to offer a caesarean section to women with a monochorionic monoamniotic twin pregnancy at the time of planned birth between 32+0 and 33+6 weeks, or after any complication requiring earlier delivery, or if she is in established preterm labour and gestational age suggests a reasonable chance of survival, unless the first twin is close to vaginal birth and a senior obstetrician advises continuing.

RCOG's patient information says the same in plainer terms: monoamniotic twins are usually born by caesarean unless you are in very premature labour.

If labour starts before the planned date

NG137 covers this too. It says to offer a caesarean if you are in established preterm labour and the gestational age suggests there is a reasonable chance of survival for the babies, unless the first twin is close to vaginal birth and a senior obstetrician advises continuing to vaginal birth. That single exception exists because there are moments when moving to theatre is the greater risk.

Because of that possibility, NG137 asks teams to have discussed place of birth and the possible need to transfer in case of preterm birth by 28 weeks at the latest, along with timing and possible modes of birth, pain relief, monitoring in labour and management of the third stage.

What to expect afterwards

NG137 says explicitly that MCMA babies will usually need to be admitted to the neonatal unit and have an increased risk of respiratory problems. That is not a possibility mentioned in passing; it is written into the recommendation about timing so that you can plan for it.

Twins Trust describes what that admission looks like in practice, including transitional care on the postnatal ward, special care units, local neonatal units and intensive care, and the fact that babies born after 32 weeks may need help keeping warm, a little more oxygen or close observation. Asking for a tour of the neonatal unit before birth is a reasonable request and is often granted.