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Fish Oil During Pregnancy May Increase Length of Pregnancy and Birth Weight

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fish oil during pregnancy

fish oil during pregnancy

In a randomized controlled trial published in the Journal of Nutrition, pregnant women who supplemented with fish oil during their third trimester had longer gestation and were more likely to have infants with higher birth weight compared to women who did not supplement with fish oil.1

Is Fish Oil During Pregnancy Safe? Study Weighs In

Despite the potential health benefits associated with omega-3s, a recent double-blind study of 736 pregnant women and their offspring sought to determine the safety of  fish oil use during the third trimester of pregnancy.

Participants from the 2010 cohort of the Copenhagen Prospective Studies on Asthma in Childhood were enrolled between 22 weeks and 26 weeks during their pregnancy. Women were randomized to either 2.4 grams of fish oil containing n–3 long-chain polyunsaturated fatty acids (LCPUFAs) or a control consisting of olive oil. Treatment was administered during the third trimester and until 1 week after birth.

Main Findings

During the study period, supplementation with n–3 LCPUFA during the third trimester was associated with a 2-day longer pregnancy duration compared to supplementation with control (median duration, 282 days [interquartile range (IQR), 275–288] vs 280 days [IQR, 273–286], respectively; p=0.02).

Use of fish oil during pregnancy, specifically in the third trimester, was also associated with a significantly higher birth weight (mean, 3601 ± 534 grams vs 3504 ± 528 grams; p=0.02) as well as an increased size for gestational age (mean, 49.9 ± 28.3 percentiles vs 44.5 ± 27.6 percentiles; p=0.01).

Should Fish Oil Be Avoided During Pregnancy?

While omega-3s are consistently touted for their health benefits, particularly for the developing brain,2 this study highlights a concern for the use of fish oil during the third trimester of pregnancy. Clinicians are advised to speak with their patients who are pregnant about the risks vs benefits of fish oil use during gestation.


  1. Vinding RK, Stokholm J, Sevelsted A, et al. Fish Oil Supplementation in Pregnancy Increases Gestational Age, Size for Gestational Age, and Birth Weight in Infants: A Randomized Controlled TrialJ Nutr. 2019;149(4):628-634. 
  2. Lauritzen L, Brambilla P, Mazzocchi A, et al. DHA Effects in Brain Development and FunctionNutrients. 2016;8(1). 
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