Essential thrombocythemia is a myeloproliferative syndrome confined to platelet production. Once the character of a megakaryocyte abnormality has been confirmed, the prognosis is good depending on thrombotic and haemorrhagic complications. The authors report the outcome of a pregnancy in a patient who had essential thrombocythemia treated by anti-aggregating drugs from the 10th week of amenorrhoea. In the previous history there were two obstetrical complications of the illness (one spontaneous abortion and one intrauterine fetal death). A caesarean section carried out after 32 weeks of amenorrhoea led to the birth of a baby weighing 1890 gm. Only a few cases have been reported in the literature. There is always a drop in the number of platelets in pregnancy. After delivery the number of platelets becomes pathological within a few weeks. The prognosis for the fetus is linked to the risk of placental thrombosis, not to the numbers of platelets. The problem lies in the choice of indications for the use of acetylsalicylic acid in low dosage, particularly since cases of pregnancy taken to term without any specific type of treatment have been reported.