Objective: To evaluate the incidence of adverse pregnancy outcomes (APOs) in women with uterine myomas and clarify the effect of uterine myomas on pregnancy outcomes. Design: Prospective cohort study Setting: The Japan Environment and Children’s Study (between 2011–2014) Population: Women (86,370) with singleton births after 22 weeks of gestation. Methods: Using logistic regression, the adjusted odds ratios (aORs) for APOs were calculated considering women without uterine myomas as the reference. Additionally, we used logistic regression to evaluate the effect of intrauterine infection (II) on the incidence of preterm births (PTB) and preterm premature rupture of membranes (pPROM). Main Outcome Measures: PTB (before 37 and 34 weeks), pPROM, II, and gestational hypertension (GH). Results: In women with uterine myomas, the aORs for PTB before 37 and 34 weeks, pPROM, II, and GH were 1.37 (95% confidence interval [CI], 1.22–1.54), 1.61 (95% CI, 1.27–2.05), 1.65 (95% CI, 1.33–2.04), 1.05 (95% CI, 0.75–1.46), and 1.20 (95% CI, 1.05–1.38), respectively. In women with both uterine myomas and intrauterine infection, the aORs for PTB before 37 weeks and pPROM were not significantly increased. Conclusions: Intra-pregnancy uterine myomas were associated with an increased risk of APOs. II in women with uterine myomas was not associated with PTB or pPROM. These data suggest a potential mechanical disadvantage in pregnant women with uterine myomas. Funding: Ministry of the Environment, Japan Key words: uterine myoma, preterm birth, preterm premature rupture of membrane, intrauterine infection