Pregnant mums urged to eat more fish, but what about mercury?

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One of the burgeoning fields in nutrition concerns the influence that maternal diet can have on the health and development of her unborn child. Every baby starts out as an egg and a sperm, and ends up, say, 6 or 7 pounds in weight. The vast majority of a baby is composed of what the mother ate during her pregnancy. Babies, essentially, are what their mother eats.

One particular food component that is believed to have an important role in the healthy development of the foetus are the so-called omega-3 fats found in oily fish such as salmon, trout, mackerel, herring and sardine. These fats are believed to be particularly important for the development of the foetal brain and visual function. This has led to recommendations that women consume a decent amount of oily fish during pregnancy.

However, in recent years, there have also been recommendations for women not to consume much oily fish during pregnancy. This is on account of the fact that some species of fish can be contaminated with mercury, which theoretically has the capacity to damage the foetus (particularly its nervous system).

This week, a group of 14 obstetricians and nutritionists from the USA urged pregnant women to eat at least 12 ounces of fish or seafood every week. They say that not doing so could rob women and their unborn children of vital omega-3. The group responsible for this pronouncement were funded – to the tune of $60,000 ” by a seafood industry trade group.

The group also allayed fears about mercury by pointing out that mercury poisoning is theoretical, and there has been so documented case of foetal mercury poisoning in the USA. The problem here is that absence of evidence does not necessarily mean evidence of absence. I am not aware of any ‘evidence’ or even a case report that getting smashed in the face with a polo mallet causes pain and suffering, though I have little doubt that it does.

The relationship between mercury and foetal development is clearly not as obvious as this, though my advice generally would be to adopt the precautionary principle. Why consume fish that is likely to be contaminated with mercury when alternatives exist? Besides, there is some evidence, admittedly from beyond the Unite States, that links higher mercury levels in mothers with neuro-developmental issues (e.g. lower IQ) in their children [1].

Below, I have added an article which covers some of science regarding the benefits of omega-3 consumption in pregnancy. It also offers some practical advice for those keen to keep up a good intake of omega-3 fat, including vegetarians.

Omega-3 fats in pregnancy
The omega-3 fats are a particular class of ‘healthy’ fats to be found in certain nuts and seeds (as alpha-linolenic acid) and in oily fish such as salmon, trout and sardine (as eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA). Research over the last 20 years has linked the so-called omega-3 fats with a range of benefits including a reduced risk of heart disease, stroke and certain forms of cancer. However, there is also a considerable amount of evidence that the omega-3 fats, DHA in particular, can have very important benefits for the growing foetus.

DHA seems to be particularly important for the visual function and the normal development of the brain. Levels of DHA in the mother tend to decline in pregnancy, which is taken as a sign that this nutrient is important for foetal development. Also, it has been noted that babies born with a low birth weight or born prematurely tend to be deficient in DHA [2].

Interestingly, higher levels of DHA are associated with healthier weight, length and head size of babies at birth [3]. Other evidence points to omega-3 fats being important for preventing premature birth and low birth weight [4].

The benefits of DHA seem to extend beyond pregnancy too. Babies born to mothers with high DHA levels in their blood have been found to have developed nervous systems, and are better sleepers [5]. And the benefits of omega-3 fats seems to extend to the mother too. Higher DHA levels during pregnancy appears to reduce the risk of post-natal depression [6].

DHA is found most abundantly in oily fish such as salmon, trout, mackerel, herring and sardine. Including two or three servings of this in the diet each week during pregnancy may have a variety of benefits. Historically, tuna has been recommended to pregnant mums on the basis of its omega-3 content. Actually, the omega-3 content is quite low. Also, tuna is one fish that tends to be contaminated with mercury, and that exposure to this during pregnancy has been linked with problems with the neurological development of the foetus. My advice is to avoid tuna during pregnancy, along with other fish that tend to be tainted with mercury such as marlin and swordfish.

Individuals who eat fish have the option of taking concentrated fish oil supplements which contain DHA. These will generally also contain another omega-3 fat found in fish known as EPA (eicosapentaenoic acid). I recommend 1 g of concentrated fish oil each day.

Fish oils are obviously not suitable for vegetarians. Another option is to supplement with flaxseed oil. This is rich in a fat known as alpha-linolenic acid which can convert to DHA in the body. However, there is evidence that this conversion is slow and inefficient, which means taking flaxseed oil does not ensure good levels of DHA in the body. However, I suggest anyone keen to take this approach should take 1 tablespoon (15 mls) of flaxseed oil a day throughout pregnancy. Another option is to use a DHA supplement that has not been derived from fish. Such supplements do exist, as DHA can also be extracted from algae.


1. Axelrad DA, et al. Dose-response relationship of prenatal mercury exposure and IQ: an integrative analysis of epidemiologic data. Environ Health Perspect. 2007 115(4):609-15

2. Crawford MA.The role of essential fatty acids in neural development: implications for perinatal nutrition. Am J Clin Nutr. 1993 57(5):703-709

3. Hornstra G. Essential fatty acids in mothers and their neonates. Am J Clin Nutr. 2000 71(5):1262-1269

4. Olsen SF, Secher NJ. 2002. Low consumption of seafood in early pregnancy as a risk factor for preterm delivery: prospective cohort study. BMJ. 324(7335):447

5. Cheruku SR et al. Higher maternal plasma docosahexaenoic acid during pregnancy is associated with more mature neonatal sleep-state patterning. Am J Clin Nutr. 2002 76(3):608-13

6. (Cheruku SR et al. Higher maternal plasma docosahexaenoic acid during pregnancy is associated with more mature neonatal sleep-state patterning. Am J Clin Nutr. 2002 76(3):608-13

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