For your sake, and the baby's sake, hold your ground. You need to gently explain (and keep explaining over and over) that being around smoke is bad for your and you baby's health. You are the person that your baby relies on to protect her, so if that means having issues with your parents, then so be it. You have a very important job now.
You might have a look at the American Lung Association's website for info. My sister-in-law works for them, and I am learning a LOT about the different things that can happen in your body due to poor air environment. These facts might help you explain things to your mom.
Great job in addressing this issue before the baby is born so you aren't imposing surprise standards on your mom when your mom really wants to be around the baby!