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To investigate the relationship of fibroids in pregnancy, preterm birth, and neonatal anthropometry. Pregnant women (n = 2578) in the National Institute of Child Health and Human Development Fetal Growth Studies–Singletons cohort had up to six ultrasounds across pregnancy. Sonographers recorded fibroid number and volume of the three largest fibroids. Trained personnel measured neonatal anthropometry. Linear and logistic regression compared neonatal anthropometry and pregnancy outcomes among pregnancies with versus without fibroids. Causal mediation analysis evaluated preterm birth as a mediator. Average birthweight did not differ by fibroid status. However, compared with pregnancies without fibroids, neonates from pregnancies with single fibroids had 0.3- (95% confidence interval [CI], 0.0, 0.5) cm larger head circumferences; those with multiple fibroids had 0.3- (95% CI, 0.0, 0.6) cm larger arm circumferences; and those with small fibroid volume had 0.7- (95% CI, 0.3, 1.2) cm larger head, 0.4- (95% CI, 0.0, 0.8) cm larger arm, and 0.7- (95% CI, 0.1, 1.3) cm larger thigh circumferences. Presence versus absence of fibroids was associated with 1.73–2.65 times higher odds of preterm birth. Differences in preterm birth did not explain fibroid-anthropometry results. We found no evidence that fibroids negatively impacted fetal growth; instead, fibroids were associated with increased head, arm, and thigh circumferences. ClinicalTrials.gov, NCT00912132. • Birthweight did not differ by fibroid status. • Based on neonatal measurements, fibroids did not appear to impede fetal growth. • Multiple and/or large fibroids were associated with higher odds of preterm birth. • Differences in preterm birth did not explain the fibroid-anthropometry results. [ABSTRACT FROM AUTHOR]