Anyone familiar with FMLA?

So I am currently 32.3 weeks, have been planning to work up until 38 weeks, using my 2 weeks of vacation, and then the six weeks FMLA gives. but my body just can not handle it any longer. I am so tired at the end of the day, And the worst part is I can not do my job very well (it's very physically demanding, on my feet 12-16 hours, lifting, etc). Is anyone familiar with stopping work earlier than delivery? What sort of protection does FMLA give? What does stopping work earlier than delivery mean on the return to work date?