Résumé : Thrombocytopenia, defined as a platelet count < 150 x109/L, is a frequent finding in pregnancy, 7% of women become thrombocytopenic at the end of gestation. A physiological decrease of 11% of the platelet counts is observed during uncomplicated pregnancy leading in some cases to thrombocytopenia. In most of these cases, thrombocytopenia is mild and devoid of any hemorrhagic risk. However several syndromes may contribute to pregnancy associated thrombocytopenia. The etiologic diagnosis is of importance for the best management of the mother, the infant and the next pregnancies. Thrombocytopenia may be isolated as seen in autoimmune thrombocytopenic purpura or associated with obstetrical manifestations such as preeclampsia and the Hellp syndrome. Thrombocytopenic thrombotic purpura and the hemolytic uremic syndrome could occasionally occur in that setting. Other potential causes of thrombocytopenia discovered during pregnancy are HIV infection, systemic lupus erythematosus, type 2B von Willebrand disease and congenital thrombocytopenia. Whatever the platelet counts, pregnant women with isolated thrombocytopenia should be carefully evaluated with a history-taking oriented to reveal previous thrombocytopenia, including prior deliveries and whether the neonate had low platelet counts or experienced bleeding complications, or to get information concerning familial cases. Physical examination and laboratory testing including search for autoimmunity are essential. In such conditions the distinction is of importance between autoimmune thrombocytopenia, which may result in fetal and neonatal thrombocytopenia, and gestational thrombocytopenia benign for the fetus. In most cases the diagnosis is made retrospectively as there is no specific testing during pregnancy to ascertain the autoimmune origin, by the follow-up of the maternal platelet counts in the post partum period and the evaluation of the neonatal platelet counts during the first week of life. In multiparous women platelet counts of the previous siblings and the results of platelet life span study performed between two pregnancies are predictive of the occurrence of a fetal thrombocytopenia. Controversies still exist concerning the best way of delivery to avoid intracerebral haemorrhages for fetuses at risk of severe thrombocytopenia.