Transient normal platelet counts and decreased requirement for interferon during pregnancy in essential thrombocythaemia

We report on the remarkable decrease in the platelet counts during pregnancy in two women with essential thrombocythaemia following treatment with recombinant interferon-α (r-IFN-α). Prior to pregnancy, the first patient was treated for 10 months with r-IFN-α 3 x 10⁶ units/d six times per week, and the platelet count ranged between 750 and 800 x 10⁹/l. Starting from the sixth week of gestation, the platelet count decreased to normal levels and remained so, resulting in a lower r-IFN-α requirement. Following successful delivery of a healthy newborn an abrupt rise of the platelet count to pre-gestation values was observed, necessitating increased r-IFN-α dosage as before pregnancy. The second patient when she became pregnant had been treated with r-IFN-α 3 x 10⁶ units/d six times per week for 10 weeks. Starting from the 24th week of gestation the platelet count decreased, and despite reduction in the dose of r-IFN-α reached normal values at the time of delivery. The exact mechanism for the platelet count normalization during pregnancy is unclear, and several possibilities are discussed.