FMLA process

Anyone who’s used FMLA before know how it works? I know I’m entitled to 12 weeks & my Dr asked me when would I like to start. I told her the 23rd of this month, I’ll be 38weeks. We filled out the forms, they just have to be submitted. Now, my question is after I submit it do I have to wait for confirmation or Can I just start my leave at my desired date once the paperwork is turned in?