Concordant open neural tube defects in a monochorionic twin pair of a dichorionic-triamniotic triplet gestation

Introduction: Myelomeningocele (MMC) is a type of open neural defect that involves exposure of the meninges and neural elements. There are fewer than 30 cases of twins with concordant MMCs reported in the literature. We discuss a case of dichorionic-triamniotic triplets in which the monochorionic-diamniotic pair were diagnosed antenatally with MMC. Case Summary: The mother presented for routine prenatal care at 11 weeks of gestation as a 31 year-old G5P4003. A limited ultrasound at 18 weeks gestation confirmed her estimated due date and three viable fetuses were observed. She was found to have dichorionic-triamniotic triplets with Triplet A and Triplet B sharing a placenta. A detailed anatomic survey demonstrated MMC of Triplet B at L4-S2 and suspected open neural tube defect in Triplet A. Triplet C had no apparent defects on ultrasound. MMC of Triplets A and B were confirmed postnatally. Chromosome microarray analyses for the triplets were normal. Discussion: Most cases of MMC in monozygotic twins are non-concordant and the role of genetics remains ambiguous. Our case suggests a possible shared genetic etiology. Future studies may investigate the potential for a single-gene mutations driving the development of MMC.