Any time we eat a raw or undercooked animal product -- an over-easy egg; raw eggs in cookie dough or mousse; unpasteurized goat, feta or Roquefort cheese; beef carpaccio, and, of course, sushi -- we risk ingesting harmful bacteria that could make us sick. It's part of eating, and the chances of eating food contaminated with something like E.coli is pretty slim.
The risk changes, though, when pregnancy comes into the picture. For someone who loves, say, salmon nigiri, the slight risk of getting sick is worth it. For someone who loves salmon nigiri and is carrying a child, the risk changes, because if mom-to-be gets sick with something like E.coli, salmonella, listeriosis or Campylobacter jejuni poisoning, it could lead to pregnancy complications, like miscarriage, or a very sick newborn baby. It's also dangerous for a pregnant woman because her immune system is suppressed (that's why flu is a greater danger, too).
It's best to avoid raw and undercooked animal products, along with deli meats, which can also contain pathogens, for the entire pregnancy to reduce the risk of acquiring foodborne illness.
Up next: speaking of contaminated food.