To our knowledge this is the first reported case of a subdural hematoma, related to fetal autoimmune thrombocytopenia, diagnosed prenatally by ultrasound. The underlying etiology was hidden maternal autoimmune thrombocytopenia. This disease, which is as serious as alloimmune thrombocytopenia, causes severe fetal thrombocytopenia, which carries with it a high risk of intracranial hemorrhage and recurrence in subsequent pregnancies. Only 14 cases of fetal subdural hematomas have been reported in the literature. The etiologies of these cases were principally traumatic, or due to disorders of hemostasis. Their prognoses were generally poor, with 50% resulting in fetal death in utero and the remaining 50% demonstrating post-natal sequelae. The presence of reverse flow in the middle cerebral artery is rare and its association with a subdural hematoma unusual. Abnormalities found on cerebral Doppler studies indicate an adaptive response to a fetal condition for which the prognosis is then very poor. Monitoring fetuses at risk for hemorrhage by ultrasound imaging and Doppler studies enables us to detect indicators of a worsening prognosis before the ultrasound appearance of morphological hemorrhage. The development of intracranial hemorrhage raises difficult management issues during the index pregnancy and in subsequent pregnancies.