Pregnancy and mercury: is it safe to eat fish?

What to eat and what not to eat during pregnancy can be a bit confusing and even contradictory. In the case of fish, there has been a lot of controversy about its consumption in pregnant women. From one side, fish is considered a good source of protein, iron, choline and one of the main sources of omega-3 fatty acids, associated with anti-inflammatory properties beneficial to health, and promoting the development of the baby’s brain (1,3). On the other side, the mercury content in fish has been linked to damage in the baby’s nervous system, risk of cerebral palsy and cognitive impairment (3,5), as well as an increased risk of developing metabolic syndrome and insulin resistance later in the baby’s life (1).

A baby's optimal growth and development before birth depends on a good environment in the womb, for example, adequate hormone production and blood flow. This environment is sensitive to maternal nutrition and her exposure to harmful compounds, such as metals like mercury (2). Mercury is an abundant compound in the earth, and its toxicity to humans depends on several factors such as dose and duration of exposure. In the fetus, mercury can be dangerous because it can cross the placenta during pregnancy and accumulate there. A prolonged exposure could lead to the accumulation of harmful amounts for the baby (2).

The US Food & Drug Administration (FDA) recommends that pregnant women or breastfeeding, to choose fish species that are considered low in mercury, as well as limit its consumption. The recommendation is to eat between 2 to 3 servings a week of fish with the lowest content of mercury or one serving a week of fish with moderate content of mercury. One serving is equivalent to 4 ounces or 110 g, approximately the palm of your hand excluding fingers. Species with low content of mercury are: Anchovy, Atlantic mackerel, clam, cod, crab, salmon, sardine, shrimp, tilapia, freshwater trout, among others (see the complete list in https://www.fda.gov/food/consumers/advice-about-eating-fish). The FDA also recommends avoiding species like king mackerel, marlin, shark, swordfish, and bigeye tuna, because of their high levels of mercury (4). The guidelines about the acceptable consumption of fish and fish species varies among governments and non-governmental organizations (2). In addition to these recommended amounts, previous studies concluded that more than one but less than 3 serving of fish per week was associated with a lower risk of developing metabolic syndrome in the newborn. However, this benefit was attenuated when the fish consumption of the mother was higher than 3 times per week (1). Fish oil supplements do not contain high amounts of mercury, therefore there is no recommendation to limit their consumption (5).

Other important guidelines regarding eating seafood during pregnancy are to avoid large, predator fish due to their higher content of mercury, as well as raw fish (e.g. in sushi) because of the potential presence of harmful bacteria or viruses (3).

In summary, if you normally eat fish and get pregnant or are planning to get pregnant, there is no reason to quit eating fish if you follow the recommendations, it can still be part of a healthy diet.

References: 1-Stratakis N, Conti DV, Borras E, Sabido E, Roumeliotaki T, Papadopoulou E, Agier L, Basagana X, Bustamante M, Casas M, Farzan SF, Fossati S, Gonzalez JR, Grazuleviciene R, Heude B, Maitre L, McEachan RRC, Theologidis I, Urquiza J, Vafeiadi M, West J, Wright J, McConnell R, Brantsaeter AL, Meltzer HM, Vrijheid M, Chatzi L. Association of Fish Consumption and Mercury Exposure During Pregnancy With Metabolic Health and Inflammatory Biomarkers in Children. JAMA Netw Open. 2020 Mar 2;3(3):e201007. doi: 10.1001/jamanetworkopen.2020.1007. PMID: 32176304; PMCID: PMC7076335. 2-Dack K, Fell M, Taylor CM, Havdahl A, Lewis SJ. Mercury and Prenatal Growth: A Systematic Review. Int J Environ Res Public Health. 2021 Jul 3;18(13):7140. doi: 10.3390/ijerph18137140. PMID: 34281082; PMCID: PMC8297189. 3. Mayo Clinic. Pregnancy and fish: What's safe to eat? https://www.mayoclinic.org/healthy-lifestyle/pregnancy-week-by-week/in-depth/pregnancy-and-fish/art-20044185#:~:text=Though%20mercury%20can%20harm%20a,a%20baby's%20healthy%20cognitive%20development. Accessed: 17.10.2022. 4. US Food & Drug Administration (FDA). Advice about eating fish. https://www.fda.gov/food/consumers/advice-about-eating-fish. Accessed: 17.10.2022. 5. NSW Food Authority. Mercury and fish. https://www.foodauthority.nsw.gov.au/consumer/life-events-and-food/pregnancy/mercury-and-fish. Accessed: 17.10.2022.