Anti-U and haemolytic disease of the fetus and newborn

The red cell alloantibody, anti-U, is uncommon but is a recognised cause of haemolytic disease of the fetus and newborn. We describe six pregnancies complicated by the presence of maternal anti-U, and review nine other well-documented cases. In these 15 cases severe haemolytic disease occurred only with titres of > or = 1/512, and titres as high as 1/4000 were not necessarily associated with significant haemolysis. We recommend that an anti-U titre of > or = 1/128 or more at > or = 17 weeks of gestation is an indication for assessment of haemolysis in the fetus. Amniocentesis is the preferred initial investigation.