Background Essential thrombocythaemia (ET) is an uncommon myeloproliferative disorder with an elevated platelet count. ET occurring in pregnancy has been reported to be mainly associated with first trimester abortion, preterm delivery, intrauterine growth retardation, placental abruption and preeclampsia.

Purpose To describe the case of a pregnant patient diagnosed with essential thrombocythaemia who could not remain untreated due to complications of the disease.

Materials and methods 35-year-old female patient diagnosed with essential thrombocythaemia untreated for months so she could try for a baby reached platelet counts of 1,118,000/mL. In the first month of pregnancy this could not be left untreated because complications can arise as such microthrombosis in blood vessels, leading to a high incidence of abortions in these patients.

The treatment requested from pharmacy services for this case was interferon alpha-2 beta (IFN α-2b) which has shown no teratogenicity compared to other alternatives such as hydroxyurea or anagrelide. Low-dose acetylsalicylic acid is contraindicated in patients with platelet counts of more than a million/mL, due to the possibility of acquired Von Willebrand syndrome.

Interferon alfa has not been approved for the indication of essential thrombocythaemia so its off-label use required approval by the hospital management.

3 MUI of IFN α-2b were administered twice weekly IV during gestation.

Results Platelet counts decreased gradually from the beginning of treatment passing from 1,118,000/mL to 680,000/mL in two weeks, then 602,000/mL in the third month, in the fourth month 530,000/mL, 487,000/mL in the fifth month, in the sixth month 462,000/mL and in the seventh month 328,000/mL. During week 29 + 2 gave birth by emergency Caesarean section because of severe preeclampsia. Despite the complications, mother and child progressed favourably.

Conclusions Essential thrombocythaemia is a difficult to treat disease in pregnancy. Sometimes you have to use drugs off-label. In this case, the IFN α-2b successfully reduced the platelet count during pregnancy.

No conflict of interest.