Try not to worry yourself Loula, the chances of getting Listeria are very, very low. Like the others have said if it is from a reputable place who cooks to order then you should be fine. I ate heaps of take away when I was preg, and yes I do have to admit there were times when I worried myself (especially the time when I ate some aioli mayo and then thought to ask if it was made fresh using raw egg and was told I shouldn't eat it) but everything was fine in the end. Yes you do need to be careful, but overall the chances of getting it are very slim so don't stress too much.