The incidence of haemolytic disease of the foetus or newborn (HDFN) has decreased considerably in Denmark since the introduction of routine administration of prophylactic anti-D immunoglobulin to RhD-negative pregnant women carrying a RhD-positive foetus. RhD-positive pregnant women are screened for irregular antibodies only in the first trimester of their pregnancy, as their risk of clinically relevant immunisation during pregnancy has been considered very low. This is a case report of severe undetected alloimmunisation causing fatal HDFN after the first trimester in a RhD-positive woman.