Essential thrombocythaemia is a rare myeloproliferative disorder that often presents with haemorrhagic or thrombotic complications. It may be detected incidentally in an asymptomatic younger adult and there are only a few case reports of essential thrombocythaemia in pregnant women. The risks posed by essential thrombocythaemia during pregnancy and its optimal management are uncertain. To determine if there is increased incidence of obstetric complications seen in women who have essential thrombocythaemia, we collected a large case series from a number of tertiary obstetric units in Australia and New Zealand. There were 30 pregnancies in 12 women who had essential thrombocythaemia. There were 17 live births (57%), 7 stillbirths (23%), 5 miscarriages (17%) and 1 ectopic (3%). Five pregnancies were complicated by placental abruption. When the outcomes of those women who received treatment with aspirin or interferon were compared to those that did not receive any treatment, there was a trend towards a higher livebirth rate (79% v. 38%, p = 0.06). Seven women were treated with aspirin and 5 had successful outcomes with no fetal complications. Four women were treated with alpha-interferon which reduced their platelet counts and all had successful outcomes with no fetal complication. These findings suggest that there is a high incidence of miscarriage, stillbirth and abruption in women with essential thrombocythaemia. Their pregnancies should be carefully monitored. Treatment with low dose aspirin and/or the use of alpha-interferon may be associated with an improved pregnancy outcome.