A randomized controlled trial of 600 pregnant women from five European countries found that dietary advice about fish consumption did not reduce mercury exposure compared to standard care, according to Gram Research analysis. Although mercury levels decreased slightly overall from 0.59 to 0.55 micrograms per gram, this decline occurred equally in both the advice group and control group, suggesting other factors like changes in fishing practices drove the improvement rather than the intervention itself.
Researchers studied 654 pregnant women from five European coastal countries to see if special advice about eating fish could reduce mercury exposure. Mercury is a toxic metal found in some fish that can harm a baby’s developing brain. While mercury levels did drop slightly over time in some countries, the study found that the dietary advice itself didn’t cause this decrease. The findings suggest that other factors, like changes in fishing practices or natural variations in fish mercury content, may have played a bigger role than the advice alone.
Key Statistics
A 2026 randomized controlled trial of 600 pregnant women across five European coastal countries found that harmonised dietary advice about fish consumption did not significantly reduce mercury exposure compared to standard care.
Mercury levels in pregnant women decreased from 0.59 to 0.55 micrograms per gram over the study period, but this 6.79 percent reduction occurred equally in both the intervention and control groups, indicating the advice itself was not responsible for the improvement.
The proportion of pregnant women exceeding the European Food Safety Authority’s mercury exposure benchmark decreased from 12.5 percent at baseline to 7.5 percent at follow-up, though this improvement was not attributable to the dietary intervention.
Portugal had the highest baseline mercury exposure among study participants at 1.56 micrograms per gram, while Cyprus had the lowest at 0.31 micrograms per gram, demonstrating substantial geographic variation in mercury exposure during pregnancy.
The Quick Take
- What they studied: Whether giving pregnant women specific advice about which fish to eat could reduce the amount of mercury in their bodies.
- Who participated: 600 pregnant women from Portugal, Spain, Greece, Iceland, and Cyprus who were followed from 2020 to 2022.
- Key finding: Mercury levels in hair dropped slightly overall (from 0.59 to 0.55 micrograms per gram), but this decrease happened equally in both the group that received dietary advice and the group that didn’t receive it.
- What it means for you: While the specific advice didn’t make a measurable difference, mercury exposure during pregnancy remains important to monitor. Pregnant women should still follow general guidelines about fish consumption, but this study suggests that simple dietary advice alone may not be enough to significantly reduce mercury exposure.
The Research Details
This was a randomized controlled trial, which is considered one of the strongest types of research studies. Researchers randomly divided 654 pregnant women from five European coastal countries into two groups: one group received harmonised dietary advice about fish consumption (designed to reduce mercury exposure), while the other group received standard care without special advice. Hair samples were collected at the start of the study and again several months later to measure mercury levels. Hair is used because mercury accumulates there over time, providing a reliable measure of exposure.
The study was conducted between 2020 and 2022 as part of the HBM4EU-MOM project, which examined how human exposure to chemicals varies across Europe. By comparing the two groups, researchers could determine whether the dietary advice specifically caused any reduction in mercury levels. The study also looked at whether results differed between countries, since mercury exposure varied significantly depending on local fishing practices and fish consumption patterns.
This research approach is important because it helps distinguish between what actually works versus what just happens to occur naturally. Many things change over time, and mercury levels might drop for reasons unrelated to advice, such as changes in fish farming practices or seasonal variations in fish mercury content. By having a control group that didn’t receive the advice, researchers could see whether the advice itself made a real difference. This type of study design is the gold standard for testing whether interventions actually work.
The study was well-designed with a reasonable sample size of 600 participants across multiple countries, which strengthens the findings. However, the study had some limitations: it only included coastal populations who eat more fish than average, so results may not apply to inland populations. Additionally, the study relied on self-reported dietary information, which can be inaccurate. The fact that mercury levels naturally decreased in some countries during the study period suggests that factors beyond the intervention were at play, making it harder to isolate the true effect of the advice.
What the Results Show
The main finding was that the harmonised dietary advice did not reduce mercury levels in pregnant women compared to standard care. When researchers compared the group that received advice to the control group, there was no statistically significant difference in how much mercury decreased in their hair samples. The adjusted analysis showed a ratio of 1.028, which means the intervention group actually had slightly higher mercury levels at follow-up, though this difference was not statistically significant and could easily be due to chance.
However, mercury levels did decrease overall across all participants. The average mercury concentration dropped from 0.59 micrograms per gram at baseline to 0.55 micrograms per gram at follow-up, a decrease of about 6.79 percent. This is a modest but measurable improvement. Additionally, the proportion of women whose mercury levels exceeded the European Food Safety Authority’s recommended benchmark (1.8 micrograms per gram) decreased from 12.5 percent to 7.5 percent.
Interestingly, this improvement was not unique to the group receiving dietary advice. Both the intervention group and the control group showed similar decreases in mercury levels, suggesting that something other than the advice was responsible for the improvement. In Spain and Portugal, where baseline mercury levels were highest, both groups showed statistically significant reductions over time. In Greece, Iceland, and Cyprus, where baseline levels were lower, no significant changes were observed in either group.
The study revealed important country-specific patterns. Portugal had the highest baseline mercury exposure with a geometric mean of 1.56 micrograms per gram, followed by Spain at 0.90 micrograms per gram. Greece, Iceland, and Cyprus had much lower levels ranging from 0.31 to 0.44 micrograms per gram. These differences likely reflect variations in local fish consumption patterns, types of fish eaten, and mercury content in regional fish populations. The fact that mercury levels decreased in Spain and Portugal but not in the other countries suggests that local factors, possibly related to changes in fishing practices or fish availability, influenced exposure more than the dietary advice did.
This study adds important nuance to the ongoing discussion about mercury exposure during pregnancy. Previous research has established that mercury can harm fetal brain development, which is why health agencies recommend limiting consumption of high-mercury fish during pregnancy. This study confirms that mercury exposure remains a concern in European coastal populations, with some women still exceeding safe exposure benchmarks. However, unlike some earlier studies that suggested dietary interventions could significantly reduce mercury exposure, this research found that simple advice alone was not sufficient. The findings align with growing evidence that reducing mercury exposure requires multi-faceted approaches beyond individual dietary counseling.
Several limitations affect how we should interpret these results. First, the study only included pregnant women from coastal countries who tend to eat more fish than average, so the findings may not apply to inland populations or countries with lower fish consumption. Second, the study relied on women self-reporting what they ate, which can be inaccurate or biased. Third, the study was relatively short (about two years), so we don’t know if effects would be different over longer periods. Fourth, the fact that mercury levels decreased naturally in both groups makes it difficult to determine whether the advice had any effect at all. Finally, the study measured mercury in hair, which reflects exposure over several months, so changes might not be immediately apparent after receiving advice.
The Bottom Line
Pregnant women should continue to follow established guidelines about fish consumption during pregnancy, particularly limiting high-mercury fish like shark, swordfish, and king mackerel. However, this study suggests that receiving dietary advice alone may not significantly reduce mercury exposure. Instead, a combination of approaches may be more effective: following fish consumption guidelines, choosing lower-mercury fish species, varying the types of fish eaten, and potentially reducing overall fish consumption if baseline exposure is high. The confidence level for these recommendations is moderate, as the study shows that other factors beyond individual dietary choices influence mercury exposure.
Pregnant women and women planning to become pregnant should pay attention to these findings, particularly those living in coastal areas or countries with high fish consumption. Healthcare providers should understand that simple dietary advice may not be sufficient for all populations and that more comprehensive strategies may be needed. Policymakers should consider these results when developing public health recommendations about fish consumption during pregnancy. Women with very high baseline mercury exposure (above 1.8 micrograms per gram) should work with their healthcare providers to develop personalized strategies to reduce exposure.
Mercury levels in the body take time to change because mercury accumulates in hair and tissues gradually. In this study, measurable changes in hair mercury took several months to appear. Women should not expect immediate results from dietary changes. Instead, consistent adherence to fish consumption guidelines over several months to a year is more likely to produce meaningful reductions in mercury exposure. The natural decrease observed in this study occurred over approximately one to two years.
Frequently Asked Questions
Does dietary advice about fish help reduce mercury exposure during pregnancy?
A 2026 study of 600 pregnant women found that dietary advice alone did not significantly reduce mercury levels compared to standard care. Mercury exposure decreased naturally over time in both groups, suggesting other factors beyond individual advice drive exposure changes.
What fish should pregnant women avoid to reduce mercury exposure?
Pregnant women should limit high-mercury fish like shark, swordfish, king mackerel, and large tuna. Lower-mercury alternatives include salmon, sardines, anchovies, and cod. Varying fish types and limiting portion sizes provides additional protection.
How long does it take for mercury levels to decrease after changing diet?
Mercury accumulates in hair and tissues gradually, so measurable changes typically take several months to appear. The study showed changes over one to two years, suggesting consistent dietary modifications over extended periods are needed for meaningful reductions.
Are mercury levels in fish the same everywhere?
No. The study found mercury exposure varied significantly by country, with Portugal at 1.56 micrograms per gram and Cyprus at 0.31 micrograms per gram. Local fishing practices, fish species availability, and water conditions influence mercury content in regional fish populations.
What percentage of pregnant women have unsafe mercury levels?
At the start of the study, 12.5 percent of pregnant women exceeded the European Food Safety Authority’s safe mercury benchmark of 1.8 micrograms per gram. This decreased to 7.5 percent by follow-up, though the improvement wasn’t due to the dietary advice alone.
Want to Apply This Research?
- Track weekly fish consumption by type and portion size. Users should log each fish meal, noting the species (salmon, tuna, cod, etc.) and amount eaten. This creates a baseline for understanding personal mercury exposure patterns and helps identify which fish types are consumed most frequently.
- Users can set a goal to replace high-mercury fish (tuna, swordfish, shark) with lower-mercury alternatives (salmon, sardines, anchovies) at least twice per week. The app can send reminders when planning meals and suggest specific low-mercury fish recipes. Users can track progress by monitoring how many high-mercury fish meals they eliminate each month.
- Establish a monthly review where users assess their fish consumption patterns and mercury exposure risk. The app can calculate estimated mercury intake based on fish type and frequency, providing visual feedback on whether consumption aligns with health guidelines. For pregnant women, quarterly check-ins with healthcare providers can include discussion of dietary patterns tracked in the app.
This research summary is for educational purposes only and should not replace professional medical advice. Pregnant women concerned about mercury exposure should consult their healthcare provider or obstetrician for personalized guidance. Mercury exposure risks and safe fish consumption recommendations may vary based on individual circumstances, local fish populations, and regional health guidelines. This study examined specific European populations and may not apply to all geographic regions or dietary patterns. Always follow your healthcare provider’s recommendations regarding diet during pregnancy.
This research translation is published by Gram Research, the science division of Gram, an AI-powered nutrition tracking app.