Women are generally advised not to eat more than two - three servings of oily fish such as salmon, tuna, mackerel and sardine per week as these types of fish are more likely to have accumulated toxins in their bodies known as dioxins and PCBs which can be harmful to your foetus if present in your blood to a large extent. Canned tuna and salmon are the exception, because the heating and canning process removes most of the toxins - you can continue to eat canned tuna as before.
You should also eat very little or avoid fish like shark, marlin and swordfish altogether because of mercury levels, and only eat shellfish which have been thoroughly cooked because of the risk of food poisoning from eating undercooked or contaminated shellfish.
You don't need to - and shouldn't - avoid eating fish altogether, because it's a great source of protein as well as essential Omegas 3 and 6. Try not to worry too much, the medical guidelines are just that - guidelines not strict rules.