Concordant partial urorectal septum malformation sequence in monozygotic twins.

The urorectal septum malformation (URSM) sequence is defined as the absence of the perineal and anal opening in association with ambiguous genitalia and urogenital, colonic, and lumbosacral anomalies. The URSM sequence is usually lethal in the newborn period due to pulmonary hypoplasia resulting fromsevere oligohydramnios. The abnormalities of this condition are though to arise early in development due to incomplete subdivision of the primitive cloaca and lackof breakdownof the cloacal membrane [Escobar et al., 1987; Wheeler et al., 1997]. A less severe form of URSM sequence is referred to by Wheeler and Weaver [2001] as the partial URSM sequence. Individuals with a partial URSM sequence typically have a single perineal/anal opening that serves as an outlet for a common cloaca and conduit for urine and feces to the outside. It is important to differentiate the partial from the full URSM sequence because the prognosis in the partial URSM sequence is generally good, with long-term survival a common feature. In this article we report a case of monozygotic twins concordant for the milder and generally nonlethal form of the URSM sequence. Monozygotic twinning with this condition has not been previously reported. The mother, a 19-year-old Caucasian woman, gravida 1, para 0, was presented to the ultrasound unit of the department of medical genetics and fetal medicine at 18 weeks of spontaneous pregnancy for routine screening. There was no family history of congenital malformations. The mother denied teratogenic exposure. On sonographic examination, a monochorionic, diamniotic twin pregnancy was diagnosed. Twin A had dilated distal bowel loops with enterolithiasis (an ultrasonographic examination revealed an echogenic bowel with multiple foci of calcified meconium intraluminally). Twin B had dilated distal bowel loops. A possible diagnosis of anal atresia was made. The genitalia could not be clearly determined. In view of the abnormal sonographic findings, amniocentesis was performed. Both fetuses had a normal 46,XY karyotype. The patient was referred to the high-risk clinic. Fetal biometry was appropriate for gestational age and a normal amount of amniotic fluid was observed. To further exclude anorectal malformation, magnetic resonance imaging (MRI) was performed which demonstrated dilated distal bowel loops (with enterolithiasis in twin A). The parenchyma of the kidneys and urinary bladder appeared normal. The twins were delivered by cesarean at 35 weeks of gestation after premature membrane rupture. TwinA weighed 1,950 g, twin B weighed 2,350 g. Postpartum examination revealed close placental insertions of both umbilical cords, each containing three vessels. Over a length of 25 cm the umbilical cords, separated by amniotic membranes, ran in such close proximity that they appeared to have a common course. The monochorionic, diamniotic twin pregnancy was confirmed and concordant fetal abnormalities were diagnosed. Both neonates had a single