Monozygotic twins discordant for external hydrocephalus

External hydrocephalus (EH) is a transient, developmental condition in infancy characterized by macrocephaly and prominence of the subarachnoid spaces. The cause is unknown, but many patients have a family history consistent with autosomal dominant inheritance. This report describes a pair of monozygotic twins, only one of whom--the recipient of a twin-twin transfusion--has EH. Whatever the genotype of the twins, their discordant phenotypes suggest that the disproportionate calvarial growth that characterizes EH is set in motion--or not--during a limited, critical temporal window in fetal development.