Understanding Monochorionic Monoamniotic (MCMA) Twin Pregnancies: Risks and Outcomes

Monochorionic Monoamniotic Twin Pregnancy

Monochorionic monoamniotic (MCMA) twin pregnancy occurs when two fetuses share one placenta and one amniotic sac. This condition arises from the splitting of a single fertilized egg into two embryos between 9 and 13 days after fertilization. MCMA pregnancies are rare, with an incidence of about 8 per 100,000 pregnancies. They make up approximately 1% of all twin pregnancies and around 5% of monochorionic pregnancies.

Complications and Risks

MCMA twin pregnancies carry a higher risk of complications compared to dichorionic or monochorionic diamniotic pregnancies. The chance of congenital anomalies is between 15% and 25%, with cardiac defects being the most common. The increased risk for these anomalies is linked to delayed embryo splitting and significant blood flow variations through the placenta.

Fetal monitoring in MCMA pregnancies is challenging due to the absence of an intertwin membrane, which leads to unpredictable fetal movements. Only 2–4% of MCMA pregnancies develop twin-to-twin transfusion syndrome, making diagnosis difficult since the typical signs, oligohydramnios, and polyhydramnios, may not be present. Preterm labor is also common, with over 55% of pregnancies delivering before 34 weeks.

Outcomes and Perinatal Loss

The fetal loss rate in MCMA pregnancies is notably high, with 30% to 40% experiencing loss before 24 weeks. The risk of death for the surviving twin after a fetal loss is significant, heightening within the first and second trimesters. Most often, if one twin dies, the other twin is likely to also suffer severe complications.

In structurally normal MCMA pregnancies, acute blood flow imbalances are frequent causes of fetal death. Umbilical cord entanglement (UCE) often complicates these pregnancies, as diagnosed instances range from 55% to 100%. UCE occurs when fetuses in one amniotic sac move around each other, making monitoring especially important.

Early Diagnosis of Cord Entanglement

Early identification of UCE is crucial as it may affect pregnancy outcomes. Very limited cases of first-trimester UCE have been documented, with only six reported in literature. This underscores the need for ultrasound examinations early in pregnancy.

This study discusses an MCMA case diagnosed with UCE at 8.5 weeks of gestation. It highlights the feasibility of early ultrasound diagnosis, currently the earliest known case of UCE in this context. This review will also encompass all reported cases of first-trimester UCE to assess its impact on pregnancy outcomes.

MCMA twin pregnancies present unique challenges and risks, making early detection and monitoring vital for potential improved outcomes.