Eleven thousand one hundred pregnant women were genotyped for human platelet antigen HPA 1, and 198 HPA 1bb women were followed in the pregnancy with quantitative assay for anti-HPA 1a antibodies. Antibodies were detected in 24 women, and nine children were born with severe thrombocytopenia (< 50×109/L). All mothers with high levels of antibodies were delivered of children with severe thrombocytopenia. None of the newborn infants had clinical signs of intra-cranial haemorrhage. The level of maternal anti-HPA 1a antibodies is predictive for fetal thrombocytopenia and may be used in decisions related to time and mode of delivery.