In a study of over 1,000 women, mothers who gave birth very prematurely were three times more likely to have high levels of mercury. Their mercury exposure was linked to fish consumption. Women who ate more fish had higher mercury levels.

Fish consumption during pregnancy--the main identified source of mercury for these women-- is often associated with beneficial effects for the baby because of omega-3 fatty acids that come with a high fish diet. This research suggests those benefits may need to be balanced against the health consequences of premature birth.

Context
What did they do?
What did they find?
What does it mean?

Context

A normal pregnancy lasts 38 - 40 weeks. When a baby is born prematurely-- before 37 weeks-- it faces a series of health problems, some of which can have life-long consequences. More babies are being born prematurely in the U.S., and many public health officials are searching for an explanation. According to the Mayo Clinic, premature births have increased by more than 30% over the last 20 years and now number 1 out of every 8 births.

Though some risk factors for premature delivery are known (such as smoking or poor nutrition), almost half of all premature births have no identified cause.

What did they do?

Fei Xue and her colleagues examine whether consumption of fish during pregnancy is linked to levels of methylmercury in the mother and subsequently to rates of preterm delivery. More than 1,000 pregnant women participating in the long-term Pregnancy Outcomes and Community Health (POUCH) study (Holzman et al. 2001) were interviewed and filled out questionnaires for this study. The women reside in five diverse Michigan communities representing urban, suburban and rural environments.

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Consequences of
premature birth

With shorter pregnancies come complications, not just at birth but for the life of the individual born prematurely. The earlier the birth, the greater are the risks.

While medical care of premature babies has improved dramatically, many very early babies simply don't survive. About 25% of infant deaths in the U.S. are linked to prematurity.

Premature infants who survive are at higher risk for infant mortality, cerebral palsy, chronic lung disease, gastrointestinal problems, mental retardation and vision and/or hearing loss, according to the U.S. Institute of Medicine.

The Institute of Medicine estimates that the health costs of carrying for babies born prematurely in the U.S. may be as much as $26 billion per year.

Participants reported the kind and amount of fish and seafood eaten during the first six months of pregnancy, including canned fish, shellfish, other bought fish, sport-caught fish and any other fish consumed.

Mercury levels in the mothers were determined from hair samples obtained prior to week 27 of pregnancy.

The authors compared women in the sample group and other women in the community at large and found that, on average, they were very similar in age, education levels, socio-economic status and rates of adverse birth events (stillbirth, preterm infant, low birth-weight infant). One significant difference was the proportion of African American: 14% in the POUCH study compared to 21% of all registered births in the community.

What did they find?

Fish consumption by the study participants was generally low, particularly compared to populations that rely on seafood as a major dietary component, such as residents of the Faroe Island in the North Atlantic Ocean who consume fish and marine mammal blubber and meat (Davidson et al. 1999).

Approximately 10% of the women ate no fish, about 25% had only 2 fish meals during the first 6 months of pregnancy, and about 50% ate more than 9 meals. Canned fish was the most frequently eaten, while fewer than 10% of the participants reported consuming sport-caught fish.

In general, mercury levels in the hair of these pregnant women ranged from 0.01 – 2.5 parts per million (ppm). These concentrations are lower than those found in “heavily exposed, fish-eating communities,” which may have levels as high as 5.9 ppm.

The authors also examined how various factors influenced the level of exposure to methylmercury. Mercury levels in women who acknowledged smoking prior to or during their pregnancy were not significantly different compared to women who claimed not to have smoked. However, higher levels of mercury were found in women older than 25 compared to women younger than 25, in women who were not insured by Medicaid compared to those who were and in women who self-identified as white or other ethnicity compared to African American women.

Within the population studied, women who ate more fish had higher mercury levels. Most women who had not eaten fish during their pregnancy had very low methylmercury levels, ranging from 0.01 to 0.12 ppm. However, a majority of the women who ate more than 24 fish meals in 6 months had moderate to high mercury levels, ranging from 0.27 to 2.5 ppm. This important finding agrees with previous published reports and further establishes fish consumption as a major source of mercury exposure in human.

Figure adapted from Xue et al.