Partial monochorionic and monoamniotic twin pregnancies: a report of two cases.

Monochorionic (MC) twin pregnancies are at increased risk of adverse outcome because of the vascular anastomoses that connect the two fetal circulations. MC monoamniotic (MA) twins are at an even higher risk because of their almost universal cord entanglement and possible compression, which can cause an acute transfusion imbalance between the twins. Chorionicity and amnionicity should be determined during the first-trimester ultrasound examination to identify high-risk MC and MA twin pregnancies for which a fortnightly follow-up may improve outcome. Although this can be achieved readily by assessing and counting the membranes that separate the twins, some pitfalls may occur. We present our observations of two monozygotic twin pairs with an intermediate type of monodichorionic and monodiamniotic twin pregnancy. The first was recognized during the first-trimester scan and the second during the second-trimester scan.