Association of uterine fibroids with birthweight and gestational age

To determine if fibroids or their characteristics are associated with birthweight and/or gestational age, and to assess the impact of race or ethnicity.

Right from the Start (2000-2012) is a prospective cohort that enrolled women from the southern US in early pregnancy. Transvaginal ultrasounds were used to measure fibroid characteristics and confirm gestational age. Date of birth and birthweight were obtained from vital or medical records. We assessed whether fibroid presence, number, type, and volume were associated with birthweight and/or gestational age using multivariate analysis of covariance, accounting for a priori confounders.

Among 3926 women, 416 had one or more fibroids. Mean infant birthweight and gestational age were similar among women with and without fibroids. When adjusting for race or ethnicity, all associations were attenuated. Overall, women with and without fibroids had infants of similar birthweight (-20 grams, 95% confidence interval [CI] -77, 36) and gestational age (0.4 days, 95% CI -0.9, 1.8). Women with three or more fibroids were more likely to have lighter infants (-201 grams, 95% CI -345, -58).

Race or ethnicity substantially confounds the associations. The clinical belief that uterine fibroids impair fetal growth is supported only by a significant decrease in birthweight for women with multiple fibroids.