Watching what you eat is never as important as when you are expecting a baby.

Researchers have discovered another important reason to watch what you eat, if you are a mum-to-be.

Philippe Grandjean of the Department of Environmental Health at Harvard School of Public Health issued a warning about becoming unwitting exposing to high levels of mercury.

His research team has discovered methylmercury, which can enter the food chain via fish, is believe to cause more brain disorders in unborn babies than previously thought, Science Daily reported.

The researchers said: "A cautious approach would involve strict regulation of suspected developmental neuro toxicants and prudent counselling of expectant mothers regarding exposures to untested substances."

People can also be exposed to methylmercury from burning waste, including mercury and coal.

The US Environmental Protection Agency issued the first advice to pregnant women about eating fish and shellfish regarding methylmercury exposure in 2004.