Telling pregnant women to eat less fish that may contain mercury causes more harm than good, say Harvard University researchers.

The US Food and Drug Administration notes that mercury can harm an unborn child's developing nervous system, and suggests pregnant women avoid fish found to have high levels, including shark, swordfish and king mackerel.

Pregnant women who ate one-sixth less fish following a 2001 government advisory experienced an 80% decline in potential intelligence benefits for their developing children, say researchers, saying benefits of fish oils for health and mental development are greater than the harm of mercury to foetuses. Similar reductions by all Americans could produce some 8,000 additional deaths per year from heart disease and stroke, according to the findings published in the Journal of Preventive Medicine.