If you're not a big sushi eater, this one is easy, but if you love a nice slab of raw salmon like I do, this one is painful.
Here's what I gathered: sushi can carry a parasite called anisakiasis which is a type of round worm. It can have you praying to the porcelain gods pretty quickly but here's where you lose me. I can't find anything where it can harm a fetus (it actually looks like the treatment is more dangerous than the round worm) and, in many places, it's actually law that the fish has to be frozen beforehand which kills the parasite. Yes, you could get dehydrated from being sick but that could happen with any type of food poisoning. Why pick on raw fish?
There's always the mercury in fish argument. Bluefin tuna which is a common fish used for sushi is often linked to high mercury levels but even then, it doesn't typically show up on the no-no list of fish to avoid while pregnant. It was the "rawness" of the fish that seemed to make everyone squeamish.
Sounds like this is just a prejudice against food that some people find "icky" and someone should be force fed their nuts on a bamboo boat for coming up with this one.
search: anisakiasis, dangers of eating sushi, sick from eating sushi