Objectives: To evaluate risks of adverse obstetric outcomes in pregnancies with myoma(s) or in pregnancies following myomectomy. Design: Retrospective cohort Setting: The national health insurance database Population: Women who had delivered in Republic of Korea, between 2004 and 2015 Methods: We analyzed the risks of adverse pregnancy outcomes in pregnancies with myoma(s) or in pregnancies following myomectomy, compared to those in women without a diagnosed myoma. Multivariate logistic regression analysis was performed. Main Outcome Measures: The adverse obstetric outcomes. Results: There are 740,675 women who had never been diagnosed with myoma (Group A), 38,402 women who had diagnosed myoma(s) but no history of myomectomy (Group B), and 9,890 women who had a history of myomectomy (Group C). Group B and Group C had significantly higher risks of cesarean section and placenta previa, compared to Group A. The risks of uterine rupture, preterm birth, and low birth weight were significantly higher in Group C (aOR 12.78, 95% CI 6.5-25.13, p < 0.001; aOR 1.64, 95% CI 1.47-1.84, p < 0.001; and aOR 1.53, 95% CI 1.39-1.68, p < 0.001, respectively), but not in Group B, compared to Group A. The incidence of uterine rupture was the highest at delivery within one year after myomectomy and decreased over time after myomectomy. Conclusions: When a woman who might become pregnant later on is diagnosed with uterine myoma, she should be counseled about the risk of myoma(s) and myomectomy on obstetric complications, especially including the significant risk of uterine rupture in pregnancies following myomectomy.