Stress is making me nauseous, just how bad is it for baby?

I'm almost 20 weeks along and having some major issues with my fiances mother. Without going into it, the stress is really getting to me and it doesn't help that she calls several times a week to complain about something new, or that we have to make the half hour drive to her house once a week for dinner but she won't come to our house to give me a rest from driving. It's been building up for a while but things got out of hand tonight, my fiance has agreed to start standing up to her but I can't stop thinking things over in my head. The stress is making me feel like I'm going to be sick, I've tried to calm down but nothing seems to be working. How bad is stress on my unborn baby, and does anyone have tips for calming down (I've tried a warm bath, a massage, ice cream...) Please help!