Everywhere says they are potentially hazardous to eat in pregnancy, but nowhere seems to be specific why. Because have "harmful bacteria and viruses" and they might contain too much mercury, is best I can find.
Leaving aside the mercury issue...
What exactly would these bad bacteria do the fetus? Do they go across the placenta? Do they cause birth defects? Do they cause food poisoning bad enough in the mother to cause her to miscarry? Can the viruses go across the placenta or cause birth defects or stillbirth?