Cord entanglement of monochorionic diamniotic twins following spontaneous antepartum septostomy sonographically simulating a true knot of the umbilical cord

Spontaneous antepartum septostomy occurring in monochorionic diamniotic twins is extremely rare. We present a case in which prenatal sonography at 26 weeks' gestation depicted a monochorionic diamniotic twin gestation with concordant fetal growth and findings suggestive of a true knot of the umbilical cord. At Cesarean delivery at 34 weeks' gestation, spontaneous antepartum septostomy with entanglement of the two separate umbilical cords was noted. This case suggests that the differential diagnosis of findings considered consistent with a true knot of the umbilical cord in monochorionic diamniotic twin gestations, should include spontaneous antepartum septostomy and umbilical cord entanglement.