The association between uterine fibroid number and size and risk of preterm birth

BACKGROUND: While the association between uterine fibroids and preterm birth is well known, data regarding whether fibroid characteristics such as number and size modify such risk are limited. OBJECTIVE: To evaluate the association between fibroid number and size and risk of preterm birth and determine whether these characteristics impact the severity of preterm birth. STUDY DESIGN: Multicenter retrospective cross-sectional study including all patients with singleton gestations who had at least one prenatal ultrasound at 18 0/7 weeks of gestation or later and delivered within a large university health system from January 1, 2019 to December 31, 2022. When fibroids were identified on ultrasound, their characteristics (i.e., number, size, and location) were documented. Patients with more than one delivery during the study period had only their first included for analysis. The primary outcome of pre- term birth, stratified by gestational age (<37, <34, <32, and <28 weeks), was assessed based on fibroid number (1, 2, >= 3) and size (largest dimension <5 cm, 5-10 cm, >10 cm) separately in patients who had prenatally detected fibroids compared to those with no fibroids. Multivariate logistic regression was performed to adjust for potential confounders. Data were presented as adjusted odds ratios (aOR) with 95% confidence intervals (CI). RESULTS: Among the 65,950 patients studied, 4,421 (6.7%) had at least one fibroid. The prevalence of preterm birth <37 weeks was 8.6%. The presence of any fibroid, regardless of number, was associated with an increased risk of preterm birth <37 weeks compared to no fibroids (1 fibroid: 12.8% vs. 8.2%, aOR 1.26, 95% CI 1.10-1.44; 2 fibroids: 12.7% vs. 8.2%, aOR 1.27, 95% CI 1.01-1.58; >= 3 fibroids: 18.5% vs. 8.2%, aOR 1.60, 95% CI 1.29-1.99). Patients with 2 fibroids were at increased risk of preterm birth <34, <32, and <28 weeks compared to those without fibroids, while patients with >= 3 fibroids were at increased risk of preterm birth <34 and <28 weeks compared to those without fibroids. The presence of at least one fibroid, regardless of the size of their largest dimension, was associated with an increased risk of preterm birth <37 weeks compared to no fibroids (<5 cm: 12.4% vs. 8.2%, aOR 1.19, 95% CI 1.04-1.36; 5-10 cm: 15.5% vs. 8.2%, aOR 1.47, 95% CI 1.23-1.75; >10 cm: 21.4% vs. 8.2%, aOR 2.26, 95% CI 1.55-3.28). Increasing fibroid size by largest dimension was associated with an increased risk of preterm birth <34 weeks. Patients with fibroids >10 cm by largest dimension were at increased risk of preterm birth <28 weeks compared to those without fibroids. There was no association between increasing fibroid size by largest dimension and preterm birth <32 weeks. CONCLUSIONS: Data from this large cohort suggest that fibroid characteristics such as number and size are associated with preterm birth and impact the severity of preterm birth. Increasing fibroid number and size is associated with a higher risk of earlier preterm birth compared to those without fibroids.