Mercury levels in women of childbearing age dropped by a third in the past decade, a survey by the U.S. Environmental Protection Agency has found.

Mercury is a neurotoxin. Research has shown that it can be harmful for a baby's growing brain and nervous system.  Children who have been exposed in the womb to methylmercury -- a version of mercury that is even more toxic than the original -- have shown impacts on cognitive thinking, memory, attention, language, and fine motor and visual spatial skills.

High mercury levels have been associated with eating certain kinds of fish, or fish from waters that are known to have mercury contamination.

But, in general, fish in a person's diet are also highly beneficial.  Fish contain high-quality protein, vitamins and minerals and omega-3 fatty acids.  They're also low in saturated fat. So officials have struggled with getting across a complex message.

Fish that are likely low in mercury include shrimp, canned light tuna, salmon, pollock, and catfish.  Those that may contain high levels of mercury include shark, swordfish, tilefish and king mackerel.

The new study of mercury in women of childbearing age compared levels from a 1999-2000 survey with levels from surveys conducted from 2001 to 2010.

The results showed that the levels dropped 34 percent. And the number of women with mercury levels above the level of concern dropped 65 percent.

The odd thing, though, was that there was no decline in the consumption of fish.

The EPA has concluded that this "suggests that women may have shifted to eating types of fish with lower mercury concentrations."

One caveat: The EPA study was based on consumption of ocean fish. "It does not reflect trends in mercury levels in communities that depend on locally caught fish for subsistence," the agency said.