I need help!! FMLA


So I was just approved for FMLA with my company but I am so confused. The doctor filled out the paperwork saying that I would be out around 6 weeks. I had initially told the company that I was going to take the full 12 weeks, but they have approved me only for the 6 weeks. I thought taking the full 12 weeks was at your own discretion and they have to give you the 12 weeks. When I asked HR about it, they only stated that they are going off what the doctor filled out on the form. Do they have the right to deny me these extra few weeks? Do i need to ask my doctor to extend it? I'm just talking about the federal FMLA. Any information would be helpful at this point! Thanks!!