Should pregnant women take omega-3 supplements? The truth is, it depends

It all began with a simple question … should I be taking this?

Despite having spent years studying nutrition prior to medical school, I didn’t have an evidence-based answer when a prenatal patient inquired regarding the utility of omega-3 supplementation during pregnancy.

While in grad school, I had reviewed the research investigating prenatal omega-3 intakes and childhood cognition. Though there was no consensus at the time, an unintended finding was later revealed. While omega-3 supplementation may not consistently influence childhood intelligence, omega-3s do significantly lower risk for preterm birth.

A Cochrane review of 26 randomized controlled trials concluded that omega-3 supplementation during pregnancy decreased risk for preterm birth so definitively that “no more randomized controlled trials are needed.” Even though more trials have been published since, the most up-to-date review continues to corroborate the finding.

The importance of this cannot be understated. Preterm birth can be devastating. It is the leading cause of infant mortality worldwide, and the leading cause of infant morbidity requiring more than $500 million per year in neonatal intensive care unit costs in Canada.

Preterm birth also has long-term health consequences including intellectual disability and increased future risk for both chronic diseases (including hypertension and diabetes) as well as psychiatric illness in adulthood.

Omega-3 and omega-6 fatty acids are essential nutrients that control inflammation in the body. Omega-3s are anti-inflammatory; omega-6s are pro-inflammatory.

When it comes to fatty acids, the old saying that you are what you eat is literally true. Research has shown that if you eat more omega-3s, you accumulate more omega-3 fatty acids in your cells and tissues. Meanwhile, if you eat more omega-6s, you accumulate more omega-6 fatty acids.

Historically, humans consumed equal amounts of omega-3 and omega-6 fatty acids. However, today, we eat 15 times more omega-6s than omega-3s.

This affects preterm birth because prostaglandin E2 – the signalling molecule that drives the uterine contractions and cervical ripening necessary to kickstart labour – is made from omega-6 fatty acids. Thus, it’s theorized that increased intakes of omega-3 fatty acids decrease risk for preterm birth by outcompeting the omega-6s and minimizing the production of prostaglandin E2.

In fact, omega-3s are so effective at stalling the onset of labour, the new guidelines for omega-3 intakes in pregnancy have an important caveat: supplementation must be stopped at 37 weeks gestation. This is because research has shown that omega-3 supplementation can increase risk for late-term or post-term pregnancies necessitating the artificial induction of labour.

The new guidelines for omega-3 intakes in pregnancy recommend that in anticipation of pregnancy, all women of childbearing age should consume 250 mg/d of omega-3 fatty acids (specifically, two types called docosahexaenoic acid [DHA] and eicosapentaenoic acid [EPA]). When intakes are sufficient at baseline, an additional 100-200 mg/d of DHA are recommended during pregnancy. This is an amount that can be obtained from roughly two servings of fatty fish (salmon, mackerel, herrings, sardines and anchovies) per week – which happens to be the same amount that’s recommended for cardiovascular health. However, for pregnant women with low omega-3 intakes – roughly 90 per cent of Canadian women – 600-1000 mg of DHA+EPA are recommended, beginning no later than 20 weeks gestation. This amount of omega-3s is unobtainable from food.

After learning this, I casually browsed drug store shelves and was surprised to find that many prenatal supplements contained no omega-3s whatsoever. Moreover, those that did frequently contained less than the recommended amounts.

This realization inspired our research, in which we utilized the Licensed Natural Health Products Database (created by Health Canada) to identify prenatal supplements that contained omega-3 fatty acids and compared the stated amount of omega-3s on the product labels to the amounts recommended in the new guidelines.

What we found in our study, published Dec. 4 in the American Journal of Perinatology, was that 61 per cent of the prenatal supplements that contained omega-3 fatty acids provided the amount that’s recommended for women with adequate intakes of omega-3s at baseline. However, as noted above, only 10 per cent of women consume adequate amounts. Thus, only 28 per cent of products provide the amount of omega-3 fatty acids that are needed by the majority of pregnant women.

Ultimately, to capitalize on the benefit of omega-3 fatty acids, many women potentially would benefit from taking an additional omega-3 supplement alongside their prenatal supplement to meet these recommended intake levels.

One way to conceptualize the benefit of omega-3 supplementation is to quantify how many women need to take omega-3s for one case of preterm birth to be prevented. This is referred to as the “number needed to treat” or NNT. For preterm birth (delivery at less than 37 weeks) the NNT is 68, meanwhile for early preterm birth (delivery at less than 34 weeks) the NNT is 52. These numbers may sound high; however, they are comparable to other mainstream obstetrical interventions such as the provision of aspirin for the prevention of preeclampsia in high-risk women. Interestingly, aspirin not only decreases risk for pre-eclampsia, but also preterm birth, with a number-needed-to-treat of 61 (for both issues). However, it should be noted that while aspirin has been demonstrated to provide these benefits in high-risk women (such as those with diabetes, hypertension, an elevated BMI or advanced maternal age), omega-3 fatty acids have been demonstrated to lower rates of preterm birth in low-risk, healthy women. Thus, the benefits of omega-3 fatty acids are generalizable to any pregnant woman, not just those at high risk.

It’s important to realize that there are no regulations to ensure that prenatal supplements contain the stated amount of each nutrient on their label. Research has shown that sometimes there are gaps between the stated level of omega-3s on the label and the actual amount in the supplement. Nevertheless, there are third-party organizations – like U.S. Pharmacopeia – that verify the accuracy of supplement labels and place their proprietary logos on products to substantiate their validity.

It’s also important to note that by virtue of their chemical composition, omega-3 fatty acids are inherently fragile and have a tendency toward oxidation that not only threatens their efficacy, but may also make them potentially harmful.

Research investigating the levels of oxidation in commercial omega-3 supplements is mixed. Some studies have shown that fish oil supplements largely meet regulatory limits. However, others conducted worldwide have reported that anywhere from 50 per cent to 83 per cent of products exceed voluntary oxidation limits. Worse yet, while one study showed that flavoured omega-3 supplements were more likely to be oxidized, another reported that there are no obvious predictors of quality (like cost, best-before date, country of origin).

One study showed that compared to fish oil-derived omega-3 supplements, algae-derived omega-3s were less likely to be oxidized. Interestingly, in our dataset, prenatal supplements typically contained algae-derived omega-3s.

All things considered, despite their high risk for oxidation and potential for inaccurate labelling, we do know for certain that omega-3 supplementation can have a meaningful benefit, as clinical trials have demonstrated efficacy with protocols that employ commercially available omega-3 supplements. To guide our patients, we reviewed the clinical trials and compiled a short list of commercially available omega-3 supplements that were employed in the studies that demonstrated benefits.

The type of omega-3 fatty acids (EPA and DHA) that have been shown to decrease risk for preterm birth are almost exclusively found in fatty fish (namely salmon, mackerel, herring, sardines and anchovies). This raises two important challenges: first, the sustainability of the world’s fisheries; second, the risk of environmental contaminants like mercury and dioxin. Many varieties of fish that are rich in omega-3s (like mackerel, herrings, sardines and anchovies) are also moderately high in mercury, meaning that according to public health guidelines, they are only safe to eat once weekly. However, from a mercury point-of-view, most types of salmon (Atlantic, Chum, Sockeye, Steelhead, Pink) are safe to eat twice daily. Dioxins are an environmental contaminant disproportionately found in freshwater fish. Since most sources of omega-3s are ocean fish, their dioxin levels are comparable to other animal products.

There’s also the issue of varying omega-3 levels in farmed fish versus wild fish. Ultimately, the amount of omega-3s in fish depends on the fish’s diet, so depending on what farmed salmon are fed, their omega-3 levels could theoretically be greater or less than their wild counterparts. In recent years, however, levels for farmed salmon reportedly have been lower. Finally, food fraud – the mislabelling of fish species – is another potential issue in the seafood sector.

There are plant-based sources of omega-3 fatty acids including flaxseeds, chia seeds and walnuts. These sources contain a different type of omega-3s called alpha-linoleic acid (ALA). The human body has a limited ability to convert ALA into EPA and DHA (the types that decrease risk for preterm birth). Though it has been shown that estrogen – a hormone whose levels increase in pregnancy – can foster enhanced converting capacity, there is no human data on omega-3 conversion efficiency in the context of pregnancy, so it is uncertain whether women can rely on these sources to meet their needs.

In the end, a simple question – should pregnant women take an omega-3 supplement – provoked a very complex answer. The truth is, it depends. For a minority of women, dietary intakes could suffice. For those forewarned, increased intake pre-pregnancy could negate the future need. Finally, for the majority of pregnant women who would benefit from omega-3 supplementation, the choice of supplement matters.