Although my job is not particularly stressful, I do deal with very intense cases and confidential matters that can be quite disturbing at times, I work closely with the police and social services.
It's not really in my nature to get stressed out by things so I guess I can't really help there, but what I did find during pregnancy was that things come and go. Some days I would feel so tired I didn't think I would make it though the day, at around 6months my back was so painful I didn't think i would make it to the 39weeks that I worked to, but I did. What would niggle one week didn't the next, you might find this week stressful but next week a breeze. I would stick it out at work, but it's in my nature to be a stubborn git that refuses to let go!
Assuming you have to go back to your job after you have had the baby you will be facing even more stresses then, so I would think it would be healthier to tackle your stress issues, and how to handle it, rather than leaving your job.