Firstly, congratulations!
Secondly, while you'll have to check with your school, I have heard of women using a baby as a reason to defer and having it being granted (anecdotally, on places like SDN and mommd.com). But my question is are you sure you want to? IMO, infants are needy, but in many ways easier than a toddler. A newborn will want to be held (or worn), or nurse/have a bottle, or sleep, but that's mostly it. A toddler will want you to drop everything you're doing (like studying) to color, read, play with blocks, etc.
Really, you are going to have as much time off home with the baby before school starts as many working moms (not everyone is really able to take all 12 weeks that FMLA promises), though if you aren't already on the waitlist for a daycare, if you need one, I think you would definitely need to defer. I had my daughter a week after finals halfway through my last two, 2-semester pre-req courses and went back to class when she was 3 weeks old. I studied for the MCAT when she was 13-16 months old, so I'm speaking from experience. If this was my first baby, I might feel different, but I would much rather start and adjust to medical school with a 2-3 month old than a 14-15 month old.
I'm also due with mine and my husband's second baby about 2 months into school. I'm hoping (and praying) that having a playmate will allow me less interrupted study time in the future.