Leave of Absence

Do I have to give a notice (say 2 weeks) before taking a maternity LOA? I know when I took a leave for covid /bereavement etc I called my HR Dept and it cleared my schedule & started immediately. I'm 31 weeks and can not work on my feet all day anymore. I was trying to make it to like 37/38 weeks like I have with my other jobs (office) in the past but I can't. So should I call now and tell them I want it to start in 2 weeks or should I just wait and when I call tell them I can no longer work & want to start it immediately? (Also stopping earlier due to covid & being higher risk working in retail, I can let them know that too)