Low mercury fish during pregnancy.

You may be wondering how eating different kinds of fish during pregnancy can affect your baby. While there are very good sources of fish that will only do good for your baby, there is also research to suggest that eating fish that is higher in mercury than its counterparts may be very bad for your baby’s brain.

How does mercury in fish affect pregnancy?

In an initial study in 2008, researchers found that, among a 341-strong population of 3-year-olds, those whose mothers ate more than two servings of fish per week during pregnancy generally performed better on tests of verbal, visual and motor development [1]. On the other hand, tests scores were lower among preschoolers whose mothers had relatively high mercury levels in their blood during pregnancy [1].

Overall, the researchers found, children whose mothers ate fish more than twice a week had higher test scores in vocabulary, visual-spatial skills and fine-motor coordination of the hands and fingers [1]. However, children whose mothers had mercury levels in the top 10 percent of the study scored more poorly than those whose mothers had lower mercury levels [1]. Only 2 percent of mothers who never ate fish during pregnancy had blood mercury levels that high, versus 23 percent of those who ate fish more than twice weekly [1].

What happens if you eat too much mercury while pregnant?

In a more recent study (2020), doctors in Europe analysed the effect of fish intake and mercury levels in the children of mothers who consumed fish in low amounts (<1 time per week), in moderate amounts (1-3 times per week, which is in accordance with health guidelines) or in high amounts (> 3 times per week) [2]. These doctors followed the metabolic development of the children for up to 13 years from birth and found that moderate fish intake in mothers significantly improved blood parameters such as weight and size measurements, blood pressure, insulin, and blood lipid levels in the children, when these children were compared to those whose mothers consumed low levels of fish [2].

Further to this, a higher risk of metabolic and inflammatory disease was seen in the children of mothers who consumed less fish and had higher mercury levels in their body [2]. Interestingly though, high intake of fish did not show any greater benefit than that seen through moderate intake by mothers [2].

High mercury fish & pregnancy

These findings add to evidence that fish can be brain-food but highlight the importance of choosing lower-mercury containing fish during pregnancy. Oily fish such as tuna, salmon and sardines contain omega-3 fatty acids, which are important in fetal and child brain development. The problem is that fatty fish are more likely to be contaminated with mercury, a metal that is toxic to brain cells, particularly in fetuses and young children. This is why it is so important to follow the recommended daily intake (RDI) guidelines, as set out by medical authorities.

What fish to avoid while pregnant

Pregnant women are advised to avoid shark, swordfish, king mackerel and tilefish. These fish are particularly high in mercury because they eat other fish and are long-lived, over time accumulating mercury in their fat tissue. Fish that are high in omega-3 but relatively lower in mercury include canned light tuna, which has less mercury than albacore tuna, and smaller oily fish like salmon. White-meat fish such as cod and haddock tend to be low in mercury but have less omega-3 than fattier fish.

References

1. Oken et al. Am J Epidemiol. 2008 Mar 28.
2. Stratakis et al. JAMA Netw Open. 2020 Mar 2;3(3):e201007.