Twin pregnancy is associated with a high risk of congenital malformations. This review covers the risk of such anomalies in both dizygotic and monozygotic twin pregnancies, and discusses current insights into the associations relating to zygosity, chorionicity and genetic factors. The pathological basis of specific malformations unique to the monochorionic twinning process, including conjoined twinning and twin reversed arterial perfusion (TRAP) sequence, is discussed in more detail, and factors contributing to the higher perinatal mortality rate in multiple pregnancies are addressed.
aDepartment of Paediatric Pathology, Great Ormond Street Hospital for Children, London, UK
bInstitute of Child Health, University College London, London, UK
Corresponding author. Address: Department of Paediatric Pathology, Great Ormond Street Hospital for Children, Great Ormond Street, London WC1N 3JH, UK. Tel.: +44 (0)20 7829 8663; fax: +44 (0)20 7829 7875.
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