Raw seafood and large fish like tile fish need to be avoided. Shellfish is fine. In fact recent studies show that pregnant women should actually be eating more low mercury fish do to the benefits of the omega 3's
The only kind of seafood you can really eat is anything farm grown or saltwater fish. Nothing raw. Ever. The doctors don't recommend you eat ocean fish due to the high levels of mercury. Even fish with low levels isnt recommended. No offense but you should know what not to eat by 20 weeks. But nonetheless, be more careful :(
Raw or cooked? Pregnant women shouldn't eat raw seafood.