What are monoamniotic twins?

Monoamniotic twins are twins within the same amniotic cavity (same sac). Because they also share a common placenta, they are monochorionic-monoamniotic (MCMA). MCMA twins represent approximately 1% of all monozygotic (identical) twins. Presumably, they result from splitting of the embryo on day 8-9 after conception.

How are MCMA twins diagnosed?

MCMA twins are diagnosed antenatally with ultrasound by noting two fetuses without an intervening dividing membrane. In addition, only one yolk sac is present. The absence of a dividing membrane allows the fetuses to move freely within the amniotic cavity. This characteristic is important to differentiate MCMA twins from TTTS with a stuck twin, in which the dividing membrane may be difficult to see.

What are the complications of MCMA twins?

MCMA twins have a increased risk for adverse pregnancy outcome, which can be as high as 40-50%. The complications result from a number of complications, including:

• Cord entanglement. Because the fetuses can move freely within the amniotic cavity, their umbilical cords ofen become entangled. Cord entanglement may result in demise of one or both twins in approximately 50% of cases.
• Increased risk for congenital anomalies. MCMA twins have an increased rate of congenital anomalies, relative to diamniotic and dichorionic twins.
• Twin-twin transfusion syndrome (TTTS). TTTS may develop in MCMA twins. The absence of a dividing membrane precludes development of oligohydramnios in the donor twin. However, polyhydramnios in the common sac will be apparent. If additional features are present, such as lack of visualization of the bladder in one of the fetuses, or abnormal Doppler studies, the diagnosis is less problematic. Nonetheless, there may be a delay in the diagnosis, which increases the chances of a poor outcome.
• Short cervical length or signs/symptoms of preterm labor. A short cervical length is defined as a cervical length of <2.5 cm via transvaginal ultrasound. Patients may have a short cervix in the absence of polyhydramnios or other poor prognostic factors. However, a short cervical length is associated with an increased risk for pregnancy loss or extreme premature delivery.

How can MCMA twins be treated?

Depending on the gestational age at which MCMA twins are diagnosed, delivery may be an option. If the diagnosis is made far from viability, different treatment options have been proposed, including:

• Expectant management. This may be associated with an increased risk of fetal demise from cord entanglement and other complications.
• Medical amnioreduction. Fetal urination may be medically reduced by the transplacental passage of a medication administered to the mother. Decreased fetal urination reduces the amniotic fluid volume, thus limiting the mobility of the fetuses within the amniotic cavity. This decreases the chances of tightening of an umbilical cord knot if present, and thus decreases the risk of single or dual fetal demise.
• Ligation and transection of the umbilical cord of one of the fetuses. In extreme cases, occlusion and transection of the umbilical cord may be necessary to avoid fetal demise from cord entanglement. This procedure was first described by Quintero and collaborators in 1997. Transection of the umbilical cord in addition to ligation is essential to prevent demise of the other twin from cord entanglement.