Fish During Pregnancy: What’s Safe?

It's important for expectant moms to get enough omega-3 fatty acids in their diet, for they're essential to the baby's visual and neurological development. Yet the best sources of omega-3 fatty acids are fatty fish, which are often high in mercury. Too much mercury in a mother's diet can cause damage to her baby's nervous system, leading to brain damage, learning disabilities and other other neurological issues.

That said, not all fish are high in mercury, so some must be safer than others. A team in Spain looked at the amounts and types of fish pregnant women ate and correlated their mercury exposure with health measures of their newborns (Am J Clin Nutr 2009;90(4):1047-55).

As part of a larger study, 554 women who gave birth in a hospital in Valencia, Spain answered detailed questionnaires regarding their diet in their third trimester of pregnancy. The types of fish they ate were grouped into different types according to the amount of mercury in the fish: canned tuna, lean fish, large oily fish and "other" (including such items as dried or smoked fish and canned sardines, etc.). When the women gave birth, samples of blood were taken from the umbilical cord and analyzed for mercury content.

With that information about how much mercury the child was exposed to in utero, the researchers looked at the baby's length and weight. Were they small for their gestational age?

The results are interestingly mixed. The researchers found that those children exposed to the most mercury weighed, on average, about 5 ounces less than those children exposed to the least mercury. They also tended to be small for their gestational age with regard to their length.

When compared to women who only ate one serving of large oily fish per week (including swordfish, mackerel and fresh tuna) those mothers who ate 2 or more portions each week were more likely to have babies who were small for their gestational age with regard to weight. On the other hand, those who ate 2 or more portions of lean fish (hake, sole or gilthead) were less likely to have babies who were small in length for their gestational age compared to those who only ate one.

Canned tuna is the big surprise here. Mothers who ate canned tuna twice a week or more had babies who weighed more than those of mothers who ate canned tuna less than once a month and their risk of having babies who were low in wieght for their gestational age was much less.

What this means for you

The jury's still out on this one, especially with regard to canned tuna. Perhaps the canned tuna in Spain has less mercury in it than the canned tuna elsewhere. Regardless, pregnancy is a time for caution as the effects of the mother's diet during pregnancy can have lifetime effects on their child. Talk with your midwife or doctor about their recommendations with regard to fish.

First posted: November 11, 2009