When you’re pregnant, keeping track of all of the things that are unsafe to eat can feel a little overwhelming.
High levels of mercury in fish is one of those exposures that has been well documented as being detrimental to pregnancy.
But what hasn’t been well studied is what effect low mercury exposure has on the fetus and what, if any, potential benefits there are to eating fish with low mercury levels during pregnancy. Because fish is the primary source of polyunsaturated fatty acids, the nutritional benefits are important to consider in addition to potential harm that may occur from mercury exposure.
Our team* set out to learn more about this, as a part of a large study that has been ongoing and evolving in Cincinnati for the last 15 years or so.
And what we found is that there is very little risk associated with eating moderate amounts of fish with low mercury content, and the benefits of eating it outweigh the potential detriments.
In our study, we collected information on what type and how much fish mothers ate during pregnancy and also measured mercury concentrations in their blood during pregnancy. We then examined their 344 five-week-old infants, using a standard neurobehavioral scale (which looks at muscle tone, reflexes, central nervous system integrity and behavioral response). We found that 84% of women ate some fish during pregnancy, and 63% ate fish 1-3 times a month or less. Tuna, shellfish and salmon were the most commonly consumed. Mercury concentrations among the women were very similar to reference levels for U.S. women.
The babies whose mothers consumed more fish during pregnancy showed better behavior and coping skills during the exam. The babies whose mothers had higher mercury concentrations, likely coming from fish, also showed better attention during the exam. There is evidence that when babies do well during this neurobehavioral test, it is linked to better outcomes when they get older. So, for instance, babies who do well on the behavioral side of the infant test are more likely to also show better behavior later in life. And a baby who is very attentive in infancy is more likely to pay better attention in school.
This means that even a very small amount of fish – 1 to 3 times a month – provided cognitive and behavioral benefits to babies, without much risk involved. We interpreted these positive benefits to be related to the polyunsaturated fatty acids contained in fish, which have been shown to benefit attention, memory and other areas of development in children.
So what should pregnant women do with this information?
- Eat more fish to support fetal growth and development. The EPA now recommends pregnant women eat 8-10 ounces a week (or 2-3 servings), because there are known nutritional benefits. Limiting fish means missing out on important nutrients that can have a positive impact on growth and development as well as general health.
- Consume the fish that are lowest in mercury. The smaller the fish, the less mercury it is likely to have. Ex: salmon, tilapia, cod, cat fish, shrimp.
- And avoid fish with high mercury levels, such as swordfish, shark, king mackerel, and tilefish from the Gulf of Mexico.
- If you don’t like to eat fish, consider a supplement. Speak with your doctor first, but fish oil pills may be a reasonable substitute.
If you’re pregnant, put fish with low levels of mercury on your “okay to eat” list. And stay tuned for more information. We’ll be continuing to study this group of mothers and their kids as they get older. We’re hoping to learn if this fish consumption leads to better IQ later in life.
* Editor’s note: Yingying Xu is a biostatistician in the division of General and Community Pediatrics and doctoral student at the University of Cincinnati who conceived and conducted this analysis.