Study of three fetuses with allo-immune thrombocytopenia, one with autopsy findings, and comparison with existing data have permitted speculation on the nature of the initial lesion within the brain. The commonest first bleeding site is probably underneath the molecular layer of the cerebral cortex, often within the temporal lobe. Small haemorrhages may appear to be (sub)pial, but expansion will lead to so-called subarachnoid haematoma. Communication with the adjacent lateral ventricle will contribute to post-haemorrhagic hydrocephalus. Occasionally, the initial haemorrhage is within the ventricle; such a pattern may lead to post-haemorrhagic hydrocephalus, often with a porencephalic component. In utero follow-up of fetuses at risk for allo-immune thrombocytopenia will have to focus on superficial cerebral haemorrhage for early detection of brain involvement.