http://www.fda.gov/oc/opacom/hottopics/mercury/backgrounder.html
U.S. Food and Drug Administration
Fish and shellfish are important parts of a healthy and balanced diet. They are good sources of high quality protein and other nutrients. However, depending on the amount and type of fish you consume it may be prudent to modify your diet if you are: planning to become pregnant; pregnant; nursing; or a young child. With a few simple adjustments, you can continue to enjoy these foods in a manner that is healthy and beneficial and reduce your unborn or young child’s exposure to the harmful effects of mercury at the same time.