The management of pregnant patients with essential thrombocythemia (ET) and polycythemia vera (PV) may be problematic. In the literature there are approximately 300 cases of pregnancies reported in ET and less than 50 pregnancies reported in PV. To reduce the effect of reporting bias, we selected articles with either > 10 pregnancies or at least six patients, and here report on the outcome of 195 pregnancies in ET and 36 pregnancies in PV patients. The live birth rate was approximately 60% in ET and 58% in PV. Spontaneous abortion during the first trimester was the most frequent fetal complication, occurring in 31% of ET pregnancies and in 22% of PV pregnancies, respectively. Major maternal complications were more frequent in PV compared with ET (44.4 versus 7.7%). Treatment with low-dose aspirin during pregnancy in ET seemed to reduce complications and also seemed beneficial during pregnancy in PV. In high-risk pregnancies, the additional use of low molecular weight heparin and/or interferon alpha should be considered. This article also describes a registry for an observational study concerning pregnancy in chronic Philadelphia chromosome-negative myeloproliferative disorders within the European LeukemiaNet. A potential management algorithm for pregnancies in ET or PV is also provided.