Pregnancy and childbirth in women with myeloproliferative neoplasms (MPN) are reported to be associated with maternal thrombosis, hemorrhage, and placental dysfunction. To assess the risks of adverse events in pregnancy in women with MPN, we performed a large population-based study using Swedish health care registers, and included all pregnancies that had reached gestational week 22 (prior to 2008, week 28) during the years 1973-2017 in women with MPN. Control pregnancies were matched 1:1 for age, calendar year, and parity. We identified 342 pregnancies in 229 women with MPN. Preterm birth was significantly increased in pregnancies in MPN, 14% compared to 4% of pregnancies in controls (p < 0.001). Correspondingly, low birth weight (<2500 g) was also significantly increased in MPN pregnancies (p = 0.042). Stillbirth was rare, with two events (0.6%) in MPN, none in controls. Maternal thrombotic complications occurred in three (1%) of the pregnancies in MPN patients, compared to none in controls. Pregnancy-related bleeding affected 14% of pregnancies in MPN and 9% in controls (p < 0.110). Cesarean section was significantly more common in pregnancies in MPN. Incidence was 12.2 per 100.000 pregnancies. In summary, preterm birth was an important complication in MPN pregnancies, while maternal complications were less common than previously reported.