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June 16, 2019  
UTERINE NEWS: Feature Story

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  • Mercury Concerns During Pregnancy

    Mercury Concerns During Pregnancy

    April 16, 2007

    By: Allison Stevens for Uterus1

    New research suggests expecting moms may need to take extra precautions when it comes to fish consumption. Researchers have found a possible association between fish consumption during pregnancy and the risk of very pre-term delivery.
    Take Action
    Mercury Content of Commonly Eaten Fish
    The Food and Drug Administration recommends pregnant women avoid fish in the “High Mercury Content” category and eat up to 12 ounces (two average meals) a week of fish that are lower in mercury.

    High Mercury Content:
  • Swordfish
  • Mackerel king
  • Tilefish
  • Shark

    Moderate Mercury Content:
  • Bass
  • Bluefish
  • Grouper
  • Lobster
  • Mackerel
  • Tuna (fresh)

    Low Mercury Content:
  • Canned Chunk Light Tuna
  • Carp
  • Halibut
  • Monkfish
  • Snapper

    Very Low Mercury Content
  • Anchovies
  • Catfish
  • Cod
  • Salmon
  • Scallops
  • Shrimp
  • Tilapia
  • Trout
  • The Research

    The paper investigating a possible relationship between fish consumption, mercury levels, and risk of very preterm birth can be found in the January 2007 issue of Environmental Health Perspectives. The research included 1,024 women participating in the Pregnancy Outcomes and Community Health (POUCH) study. Participants were from 52 prenatal clinics in five Michigan communities and were in the 15th to 27th week of pregnancy.

    Study participants were asked which type of fish they consumed and how often. Then, mercury levels were evaluated using a hair sample. As was expected, the women who consumed greater amounts of fish had higher levels of mercury.

    Researchers found that participants who delivered their babies very pre-term (defined as < 35 weeks), were three times more likely to have had high mercury levels than those who delivered at term (defined as > 37 weeks). With preterm deliveries, the baby’s organs often do not have enough time to fully develop, which is why a full-length pregnancy is desirable.

    This is the first study to look at mercury levels and risk of pre-term birth. However, it is important to keep in mind that previous research has shown various benefits to eating fish during pregnancy, primarily due to the healthy fat content of fish.

    Exactly why high mercury levels may result in pre-term delivery is still under investigation. However, the researchers from this study speculate that it may be the oxidative stress that mercury produces at a cellular level. Mercury can also influence the shape and levels of platelets in the blood, which may somehow play a role.


    Although these results are promising, there are some limitations, as researcher Claudia Holzman points out. “We cannot rule out that the observed association was in fact due to other contaminants found in fish high in mercury.” For example, some fish are high in PCBs and other contaminants besides mercury which, in excess, may be harmful to health.

    What This Means for You

    When trying to interpret the results, Dr. Holzman reminds us that, “results from a single observational study cannot adequately inform women and clinicians about best practices in pregnancy. This observational study helps to raise awareness that a larger body of research is needed to explore these issues.”

    A Balancing Act

    In addition to mercury and other contaminants that pose a potential risk, fish is high in many healthful nutrients. For example, fish contains omega 3 fatty acids, thought to be beneficial to developing the unborn child’s brain. Fish is also a good source of high quality protein. So, how do pregnant women find a balance of consuming enough fish to receive its benefits, but not too much that they may be ingesting an unsafe amount of mercury?

    One approach suggested by Dr. Holzman is to: “eat fewer meals of fish known to have high mercury levels and more meals of fish with lower mercury levels.”

    For more information about mercury levels in fish please visit the FDA’s Food Safety website or the
    Environmental Protection Agency’s Fish Advisory website.

    Last updated: 16-Apr-07


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