What is the proper way to inform your employer that you will not be returning to work after your child is born and when should it be done?

My boyfriend and I have decided that we both like the idea of me becoming a stay at home mother rather than going back to work after our baby is born. My insurance allots 4 weeks of maternity leave and I don’t know when or how I should inform my employer. I would also like to continue my insurance as long as I possibly can before joining my boyfriends. I don’t want them to drop my coverage as soon as I tell them. Thank you for your answers!