The passage of foetal blood into maternal circulation is termed a foetomaternal haemorrhage (FMH). Most cases are clinically insignificant. However, in some cases a large FMH causes serious anaemia in the newborn child. Flow cytometry is a precise and fast method to analyse maternal blood for FMH and can be used both antenatally and postnatally. The clinician should consider using the analysis in cases of unexplained anaemia. We describe a case with severe FMH.