I took them with my first pregnancy. I didn't feel any different just wasn't in as much pain any more. I didn't get light headed or sick to my stomach or foggy or anything, but, everybody is different. Best way to do it is try maybe half if you have a low tolerance for mess and see how it makes you feel of your worried about it before taking a full dose. Or read the slip that came with your mess about the possible effects of it.
You will feel fuzzy in the head and likely sleepy.
I've taken them before when I had pnumonia. Also taken heavier things all the way up to morphine (always prescribed by my doctor/surgon). I always tried to come off the meds as quickly as possible because I dont like feeling under the influence. If your doctor is aware of your pregnancy and all of your symptoms then they have weighed the pros and cons and prescribed you the pills for good reason. if you are still doubtful then make sure to get a second opinion. Take your prescription to your local pharmacy and ask the pharmacist lots of questions about your concerns.
Hope you find relief for your migraines, I get them too and they are no joke.