Texas FMLA

Sarah
I work at walmart in the vision center as an optician. I've worked there for 2 years and have known the whole time that I am eligible for 12 weeks of unpaid FMLA. Well this month I had the walmart fmla people send paper work to my Dr and they put down "from January 23rd (my due date) to March 3rd. Subject to change after delivery" which is only 6 weeks. Who decides how much time I get? Because I know legally I am able to take 12 weeks off from my job without them replacing me but can my Dr. Decide if I take less than that? My husband and I have been saving like crazy just so I could take the whole 12 weeks. And it seems like every other women in the world is able to take off however much she asks for as long as it's under 12 weeks.