How long do I get off work when my child is born?

I am currently 37 weeks pregnant. I have a c-section scheduled in 2 weeks and have had the fmla paperwork completed, turned in and approved since july. I have since only used one day of fmla where my employer sent me to my doctor for feeling light-headed and dizzy..so I took the rest of that day off. I was under the impression that I had 12 weeks that I was entitled to take after my baby was born but today HR says no. She said that they go by what the doctor writes on the paperwork for when I can return to work. I thought that what the doctor put for returning to work only affected my short term disability pay. For example..if my doctor says I can return to work in 6 weeks..I would receive the short term disability pay up until that point but still have an additional 6 weeks that I could take unpaid. I didn't think that that meant I would HAVE to go back to work in 6 weeks. How long am I allowed to take after the birth of my child???