However, about 8 percent of women have blood levels of mercury higher than the acceptable limit set by the U.S. Environmental Protection Agency, the study warns.
"Overall, mercury levels are low in the U.S. population," says study author Susan Schober, a senior epidemiologist from the National Center for Health Statistics.
But she cautions that pregnant women and women who plan to become pregnant should follow the U.S. Food and Drug Administration (FDA) guidelines on fish consumption, as well as any state and local advisories on the amount of fish that is safe to eat, because fish is a major source of mercury exposure.
The study appears in the April 2 issue of the Journal of the American Medical Association.
Mercury exposure comes from both natural and man-made sources. Humans are exposed to mercury through coal burning, the incineration of medical waste, dental fillings and workplace exposure.
However, one of the primary sources of mercury in the body, Schober says, is the consumption of fish.
When mercury interacts with microbes in the water, it changes into methylmercury, a known neurotoxin. Methylmercury is particularly dangerous to developing fetuses, the study says.
Schober and her colleagues conducted physical examinations and took blood samples to test for mercury levels in 705 children between the ages of 1 and 5. They also tested 1,709 women between the ages of 16 and 49. And they interviewed the study participants -- or their parents -- about their consumption of fish or shellfish during the previous 30 days.
Most of the women and children had acceptable levels of mercury in their blood. Nineteen percent of the children and 6 percent of the women in the study had undetectable levels of mercury in their blood.
The researchers did find that 8 percent of the women had mercury levels higher than what is considered safe by federal standards: Women, on average, had three times the level of mercury in their blood that the children did.
Mercury levels were almost four times as high in women who ate three or more servings of fish a week, compared to those who reported eating no fish.
"We've been talking about mercury levels in children and pregnant mothers for a long time," says Dr. Mark Werner, an obstetrician at William Beaumont Hospital in Royal Oak, Mich.
Werner says he limits his pregnant patients to three servings of fish per month.
"The problem is, what is really safe to eat anymore?" adds Werner, noting that fruits and vegetables are sprayed with potentially harmful pesticides and that meat often harbors unsafe bacteria.
The FDA recommends that pregnant women avoid eating swordfish, shark, tilefish and king mackerel altogether. Other fish and shellfish should be limited to no more than 12 ounces per week. The Canadian Food Inspection Agency adds fresh and frozen tuna to that list, but says canned tuna is probably fine because those fish are raised differently.
Schober is quick to point out that "fish is an important food that has a lot of nutritional benefits, so we certainly don't want people to be so concerned that they don't eat it." In fact, she says, the American Heart Association recommends eating two servings of fish a week.
Some fish are likely safer than others, the new study reports. Haddock, tilapia, salmon, cod, pollock and sole appear to have lower levels of methylmercury, according to the study.
Schober says it's important to pay attention to local and state advisories as well the FDA guidelines, because some areas of the country may have higher mercury pollution than others.