Some types of fish contain a high level of mercury which can damage the developing nervous system of an unborn baby.

http://www.patient.co.uk/health/planning-to-become-pregnant

Some types of fish contain a high level of mercury which can damage the developing nervous system of an unborn baby. Because of this, the Food Standards Agency (FSA) advises:

• You should not eat any shark, marlin, or swordfish.
• Limit tuna - eat no more than four medium-sized cans (drained weight = 140 g per can), or two fresh tuna steaks per week (weighing about 140 g cooked or 170 g raw).

Also, do not eat raw fish or uncooked shellfish (which may contain bacteria, viruses or parasites).