Anti-D was detected for the first time at 36 weeks gestation in a group A RhD-negative primigravida without prior sensitizing events or anti-D administration. Simultaneously, foetal ultrasound showed middle cerebral artery maximum velocity (MCA Vmax) at the 95th centile but no hydrops. Labour was induced at 37 weeks. Although neonatal anaemia was anticipated, the baby, who was group O RhD-positive, was born with a normal haemoglobin and negative direct antiglobulin test and antibody screen. Retrospectively, the maternal anti-D proved to be exclusively immunoglobulin M (IgM), which does not cross the placenta. Foetal Doppler MCA Vmax measurements can give false positive results especially late in the third trimester. Thus, antibody class, especially in first pregnancies without prior sensitizing events, and the timing of foetal Doppler MCA velocity measurements should be considered when evaluating the likelihood and severity of haemolytic disease of the foetus and newborn.