Flaxseed Consumption During Pregnancy
Flaxseed should be avoided during pregnancy due to evidence of reproductive harm in animal studies, including effects on birth weight, hormonal disruption, and altered reproductive development in offspring.
Evidence of Reproductive Harm
The strongest evidence comes from controlled animal studies demonstrating significant reproductive effects:
Maternal feeding of 10% flaxseed during pregnancy and lactation lowered birth weight and produced estrogenic effects in offspring, including shortened anogenital distance in females, earlier puberty, lengthened estrous cycles, and increased sex organ weights in males 1
Flaxseed lignans transfer to offspring through breast milk, as demonstrated by radioactivity recovery in nursing pups from dams given labeled secoisolariciresinol diglycoside (SDG), the primary lignan in flaxseed 1
The lignan component SDG is responsible for these reproductive effects, producing similar results to whole flaxseed at equivalent doses 1
Mechanism of Concern
Flaxseed contains the highest concentration of plant-based lignans (secoisolariciresinol diglycoside), which are converted to mammalian lignans (enterodiol and enterolactone) with estrogenic or anti-estrogenic properties 2:
- These compounds can cross the placenta and reach the fetus 3
- They may induce hormonal imbalances affecting conception, pregnancy maintenance, and fetal development 3
- The fetal period appears to be the most hormone-sensitive developmental window 4
Bleeding Risk Considerations
Beyond reproductive concerns, flaxseed poses additional risks:
- Flaxseed oil decreases platelet aggregation and has been associated with increased bleeding risk 5
- This is particularly concerning during pregnancy when bleeding complications can threaten both maternal and fetal outcomes
Contrast with Dietary Fiber Recommendations
While increasing dietary fiber during pregnancy is recommended for constipation management (approximately 30 g/day from fruits, vegetables, whole grains, and legumes) 5, flaxseed is not mentioned as a recommended fiber source in pregnancy guidelines 5, 6, 7:
- Safe fiber sources during pregnancy include psyllium husk and methylcellulose, which lack systemic absorption 5
- These alternatives provide fiber benefits without the hormonal effects of flaxseed lignans
Expert Consensus
A comprehensive review of medicinal plants during pregnancy does not specifically list flaxseed among plants to avoid 3, and one cancer nutrition guideline notes insufficient data to make recommendations about flaxseed 5. However, a systematic review of flaxseed lignan health effects explicitly states that "animal studies suggest that pregnant women should limit their exposure" 2.
Clinical Bottom Line
The prudent approach is to avoid flaxseed consumption during pregnancy given the demonstrated reproductive effects in animal models, the ability of lignans to cross the placenta, and the availability of safer alternatives for dietary fiber supplementation. If a patient has been consuming flaxseed, discontinuation is recommended, and alternative fiber sources (psyllium, methylcellulose, fruits, vegetables, whole grains) should be substituted 5.