Prenatal omega-3 fatty acid supplementation, particularly docosahexaenoic acid and eicosapentaenoic acid, has been associated with greater birthweight in clinical trials; however, its effect on fetal growth throughout gestation is unknown.

This study aimed to examine the association between first-trimester docosahexaenoic acid and eicosapentaenoic acid supplementation and growth trajectories of estimated fetal weight and specific fetal biometrics measured longitudinally from the second trimester of pregnancy to delivery.

In a multisite, prospective cohort of racially diverse, low-risk pregnant women, we used secondary data analysis to examine fetal growth trajectories in relation to self-reported (yes or no) first-trimester docosahexaenoic acid and eicosapentaenoic acid supplementation. Fetal ultrasonographic measurements, including abdominal circumference, biparietal diameter, femur length, head circumference, and humerus length, were measured at enrollment (8–13 weeks) and up to 5 follow-up visits. Estimated fetal weight and head circumference–to–abdominal circumference ratio (a measure of growth symmetry) were calculated. Fetal growth trajectories were modeled for each measure using a linear mixed model with cubic splines. If significant differences in fetal growth trajectories between groups were observed (global P<.05), weekly comparisons were performed to determine when in gestation these differences emerged. Analyses were adjusted for maternal sociodemographics, parity, infant sex, total energy consumption, and diet quality score. All analyses were repeated using dietary docosahexaenoic acid and eicosapentaenoic acid intake, dichotomized at the recommended cutoff for pregnant and lactating women (≥0.25 vs <0.25 g/d), among women who did not report supplement intake in the first trimester of pregnancy were repeated.

Among 1535 women, 143 (9%) reported docosahexaenoic acid and eicosapentaenoic acid supplementation in the first trimester of pregnancy. Overall, first-trimester docosahexaenoic acid and eicosapentaenoic acid supplementation was associated with statistically significant differences (P-value <.05) in fetal growth trajectories during pregnancy. Specifically, estimated fetal weight was larger among women with docosahexaenoic acid and eicosapentaenoic acid supplementation than among those without supplementation (global P=.028) with significant weekly differences in median estimated fetal weight most apparent between 38 to 41 weeks of gestation (median estimated fetal weight difference at 40 weeks of gestation, 114 g). Differences in fetal growth trajectories for abdominal circumference (P=.003), head circumference (P=.003), and head circumference–to–abdominal circumference ratio (P=.0004) were also identified by supplementation status. In weekly comparisons, docosahexaenoic acid and eicosapentaenoic acid supplement use was associated with larger median abdominal circumference (changed from 2 to 9 mm) in midpregnancy onward (19 to 41 weeks), larger median head circumference between 30 to 33 weeks of gestation, and smaller median head circumference–to–abdominal circumference ratio in the second and third trimesters of pregnancy. There was no specific weekly difference in fetal femur length or humerus length by docosahexaenoic acid and eicosapentaenoic acid supplementation. First-trimester dietary sources of docosahexaenoic acid and eicosapentaenoic acid among women with no first-trimester docosahexaenoic acid and eicosapentaenoic acid supplementation (n=1392) were associated with differences in fetal biparietal diameter (P=.043), but not other metrics of fetal growth. At the recommended dietary docosahexaenoic acid and eicosapentaenoic acid levels compared with below-recommended levels, biparietal diameter was larger between 38 to 41 weeks of gestation.

In this racially diverse pregnancy cohort, first-trimester docosahexaenoic acid and eicosapentaenoic acid supplementation was associated with significant increases in fetal growth, specifically greater estimated fetal abdominal circumference in the second and third trimesters of pregnancy.