I think it's mostly common sense. If there's a chance of cross-contamination then avoid it. If there's unpasturised dairy in it, avoid it. If it's take-out or smorgasboard and you don't know how long it's been there then avoid it. Otherwise generally you can eat normally.

Keep in mind that listeria only occurs in roughly 1 in 20000 pregnancies - you are more likely to be hit by a car!