Essential thrombocythemia in pregnancy.

A 32-year-old women with essential thrombocythemia and a history of bleeding completed a full-term pregnancy. During pregnancy, the platelet count declined markedly to near-normal levels, but returned to prepregnancy levels within two weeks after delivery. The clinical course was uneventful, except for two brief episodes of vaginal bleeding during the second trimester. Possible etiologic mechanisms for pregnancy-related reduction of platelet levels in thrombocythemia include increased platelet consumption or down-regulation of platelet production by a placental or fetal factor.