...by law you are guaranteed to a similar position when you are back after 12 weeks. But I heard a colleague's wife was fired during FMLA by her company and ended up with a lawsuit. Talk to your manager first.
From my own experience, it will be tough to prepare for an exam while taking care of a baby. It will be worse if you have to work as well. Take good care of yourself and baby first. Maybe you can take your exam and ask for your raise later. Good luck.