A friend of mine is well along in her pregnancy and was instructed to
avoid not only raw fish, but soft cheeses as well. The sense I get from
speaking with her is that doctors tend to take a "why chance it?"
attitude. Sadly, a lot of this may be due to a fear of litigation, but
with so much unknown about fetal development it appears that doctors
take an "err on the side of caution" approach. It also may be that it
is easier (for the doctor to say and easier for the mother to remember)
to say "avoid all sushi" rather than, "try to limit consumption of fish
A to x amount per week, and fish B to y, oh and if you live in this
part of the country, avoid C, D and E etc."

-Tmo