Natural history of fibroids in pregnancy: National Institute of Child Health and Human Development Fetal Growth Studies - Singletons cohort

Objective: To describe the natural history of fibroids in pregnancy in a racially diverse cohort and explore whether fibroid changes were associated with participant characteristics.

Design: Prospective cohort study.

Setting: Twelve clinical sites.

Patient(s): Pregnant women (n = 2774; 27% non-Hispanic White, 28% non-Hispanic Black, 29% Hispanic, 17% Asian/Pacific Islander) who had up to 6 obstetric ultrasounds in gestational weeks 10-41.

Intervention(s): Sonographers recorded fibroid number and volume of the 3 largest fibroids at each visit. Generalized linear mixed models estimated the trajectories of fibroid number and total volume (overall and stratified by total volume at first visualization: equivalent to a fibroid of <1 cm [small], 1 to <3 cm [medium], or ≥3 cm [large] in diameter). We tested the interactions between the trajectories and race/ethnicity, age (<26, 26-30, 31-34, and ≥35 years), body mass index (<25, 25-29.9, and ≥30 kg/m²), previous miscarriage, parity, and fetal sex, adjusted for total volume at first visualization.

Main outcome measure(s): Average change in total fibroid volume during pregnancy.

Result(s): Overall, 9.6% (266/2,774) of women had a visualized fibroid at any time during pregnancy, including 9% (67/745) of non-Hispanic White women, 14% (106/770) of non-Hispanic Black women, 6% (47/794) of Hispanic women, and 10% (46/465) of Asian or Pacific Islander women. The mean total fibroid volume decreased by 1.0% (95% confidence interval [CI], -1.9%, -0.2%) per week, with a variation in starting total volume. On average, the total volume increased by 2.0% (95% CI, -0.3%, 4.5%) per week among women with small volume; decreased by 0.5% (95% CI, -2.0%, 1.0%) per week among women with medium volume; and decreased by 2.2% (95% CI, -3.4%, -1.0%) per week among women with large volume at first visualization. The volume change also varied by race or ethnicity, parity, age, and miscarriage history. For example, non-Hispanic Black women's total fibroid volume decreased more than those of non-Hispanic White, Hispanic and Asian/Pacific Islander women (-2.6%, 0.1%, 0.5%, and 0.9% average change per week, respectively). The visualized fibroid number declined on an average by 1.2% per week (95% CI, -1.9%, -0.5%) without significant variation by demographic characteristics.

Conclusion(s): The total fibroid volume declined on average throughout pregnancy. However, summarizing across all fibroids disguises substantial heterogeneity by starting total fibroid volume and maternal characteristics. The findings may be a useful reference for clinicians to anticipate how fibroids may change in obstetric patients.

Clinical trial registration number: NCT00912132.