Various types of chorionicity and amniosity (how the baby's sac looks) in monozygotic (one egg/identical) twins as a result of when the fertilized egg divides

Monochorionic twins are monozygotic (identical) twins that share the same placenta. If the placenta is shared by more than two twins (see multiple birth), these are monochorionic multiples. Monochorionic twins occur in 0.3% of all pregnancies.^[1] 75% of monozygotic twin pregnancies are monochorionic; the remaining 25% are dichorionic diamniotic.^[2] If the placenta divides, this takes place after the third day after fertilization.^[2]

[edit] Amniocity and zygosity

Monochorionic twins generally have two amniotic sacs (called Monochorionic-Diamniotic "MoDi"), but sometimes, in the case of monoamniotic twins (Monochorionic-Monoamniotic "MoMo"), they also share the same amniotic sac. Monoamniotic twins occur when the split takes place after the ninth day after fertilization.^[2] Monoamniotic twins are always monozygotic (identical twins).^[3] Monochorionic-Diamniotic twins are almost always monozygotic, with a few exceptions where the blastocysts have fused.^[2]

[edit] Complications

In addition to a shared placenta, monochorionic twins also have their circulatory systems intermingled in random and unpredictable circulatory anastomoses. This can cause disproportionate blood supply, resulting in twin-to-twin transfusion syndrome (TTTS) in 20%^[1] of MoDi pregnancies. This is the main complication of monochorionic twins.

The 80% that of MoDi pregnancies without TTTS still have high rates of birth weight discordance, fetal growth restriction, prematurity and resultant cesarean section deliveries.^[1] One twin may also fail to develop a proper heart and become dependent on the pumping activity of the other twin's heart, resulting in twin reversed arterial perfusion.^[2] If one twin dies in utero, blood accumulates in that twin's body, causing exsanguination of the remaining twin.^[2]

In the case of monochorionic-monoamniotic twins the risk of complications is substantially higher because of additional potential umbilical cord entanglement and compression.^[3] However, the perinatal mortality of monochorionic twins is fairly low.^[1]

[edit] See also

• twin reversed arterial perfusion

[edit] References