Diagnosis of umbilical cord entanglement of monoamniotic twins by first-trimester color Doppler imaging.

Monoamniotic twins occur in approximately 5% of monochorionic twin gestations as a result of splitting of the inner cell mass at 8 or more days after fertilization, and they are associated with high perinatal mortality rates, ranging between 28% and 47%. These gestations have unique pathologic conditions, including conjoined twins, a high prevalence of discordancy for fetal structural anomalies (the former and possibly also the latter resulting from consequences of the twinning process), and cord entanglement. We present a case in which monoamniotic twins at first-trimester transvaginal sonography were confirmed by color Doppler imaging as having entangled umbilical cords.