- To determine the rate, obstetric characteristics and perinatal outcome of pregnancies with uterine leiomyomas. A population-based study comparing all singleton deliveries between the years 1988 and 1999 in women with and without uterine leiomyomas was performed. Patients lacking prenatal care were excluded from the analysis. Multivariable analysis, adjusting for possible confounders, such as maternal age, parity and gestational age, was performed to investigate associations between uterine leiomyomas and selected outcomes. There were 105,909 singleton deliveries with 690 (0.65%) complicated by uterine leiomyomas during the study period. Using a multivariable analysis, the following conditions were significantly associated with uterine leiomyomas: nulliparity (odds ratio [OR]=4.0, 95% confidence interval [CI] 3.3-4.7, P<.001), chronic hypertension (OR=1.9, 95% CI 1.6-2.4, P<.001), hydramnios (OR=1.5, 95% CI 1.2-2.0, P<.001), diabetes mellitus (OR=1.4, 95% CI 1.1-1.7, P=.001) and advanced maternal age (OR=1.2, 95% CI 1.1-1.2, P<.001). Higher rates of perinatal mortality (2.2% vs. 1.2%, OR=1.8, 95% CI 1.1-3.2, P<.001) were found in the uterine leiomyoma group as compared to the control group. While adjusting for maternal age, parity, gestational age and malpresentation, pregnancies with uterine leiomyomas had higher rates of cesarean deliveries (OR=6.7, 95% CI 5.5-8.1, P<.001), placental abruption (OR=2.6, 95% CI 1.6-4.2, P<.001) and preterm deliveries (<36 weeks' gestation, OR=1.4, 95% CI 1.1-1.7, P=.009) as compared to pregnancies without uterine leiomyomas. Conversely, no significant differences were noted regarding perinatal mortality (OR=1.4, 95% CI 0.7-2.8, P=.351) after controlling for maternal age, parity and gestational age using a multivariable analysis. Uterine leiomyomas increase the risk of adverse pregnancy outcomes, thus emphasizing the importance of appropriate intrapartum management of these high-risk pregnancies.