Marlin, shark and swordfish

Pregnant women are advised not to eat the above fish because of their high mercury content and the risk of neurological damage that this poses to the fetus. They should also limit the amount of tuna that they consume to no more than one tuna steak or two medium size cans of tuna per week (total 140g cooked fish). Other fish contains much lower amounts of mercury and is important as part of a balanced healthy diet.

http://www.doh.gov.uk/cmo/urgentarchive/cmo02_06.htm

http://www.foodstandards.gov.uk/healthiereating/asktheexpert/pregnancy/#h_112259