WASHINGTON (Reuters) - Women who want to become pregnant, are pregnant or breast-feeding should eat at least 12 ounces (about 341 g) of fatty fish such as tuna every week to help themselves and their babies, experts will recommend later on Thursday.

Fish including mackerel, sardines, light tuna and salmon are rich in omega-3 fatty acids -- components of fat known to help brain development. Walnuts, flaxseed oil and leafy green vegetables also contain the compounds.

Women need the nutrients to prevent postpartum depression and babies need them for brain and motor skill development, the National Healthy Mothers, Healthy Babies coalition says.

The coalition, which includes the American Academy of Pediatrics, the National Institute on Child Health and Human Development and the Centers for Disease Control and Prevention, says "90 percent of women are consuming less than the recommended amount of fish."

But these fish can also carry high levels of mercury, which is a brain and nerve toxin.

Because of this, the U.S. Food and Drug Administration and Environmental Protection Agency issued consumer advisories in 2001 and 2004 advising women who were pregnant, breast-feeding or trying to become pregnant, as well as young children, to eat no more than 12 ounces weekly of certain types of fish.

Estimates on the dangers posed by mercury come from people exposed in chemical spills. No major studies have shown evidence mercury from food or vaccines has caused brain damage to mothers or children.

Michael Bender, director of The Mercury Policy Project, an advocacy group that believes mercury exposure has damaged children, said women should choose fish not likely to be contaminated with mercury.

"While it's recognized that fish is an important source of protein, especially for pregnant women, this new emphasis on eating more than 12 ounces of fish per week, without mention of the need to avoid mercury contaminated fish, appears to throw the baby out with the bath water," he said in a statement.

The FDA and EPA "recognized that there is enough mercury in certain fish to pose health risks, especially for heavy and moderate fish consumers, women of child-bearing age and children.

Mercury is released mainly in industrial and power-plant emissions. It settles into lakes and oceans and builds up in the flesh of fish and the animals that eat the fish.

Carnivores such as tuna and sharks thus carry high levels of mercury in their flesh.