Asking boss to work part

Hi! I'd like to work part time after my baby is born. I'm due in 10 weeks and have planned a 12-week maternity leave. Do you recommend my approaching my boss about part-time availability now or during my first week back to work? I'd like to know if it's a possibility now so that I can work on lining up day care. However, I was wondering if you all think it's best to wait so I don't jeopardize anything now. Thanks!