Hr told me to book my maternity leave

Hr told me too book my maternity leave early I told them tht I wanted to wait till I give birth to apply but they say that no one does that. They force me to give a date so I just told them I'll be starting it on my due date then. My gynae has told me tht I won't go above my due date as I have gestational diabetes. But since I book the date for maternity leave on my due date do I have to inform them I gave birth earlier still or just let them start my maternity leave on my due date? I still have a lot of annual leave that I apply before due date. N my job is a retail job but some how it's so hard to use my annual leave. By the time I come back from maternity leave it will be block period meaning no one can use annual leave till end of year. Then starting of the new year I will only have 3 month to clear my leave before it's burn. In tht 3 months it's hard to take long leaves as my annual leave can't clash with others. I'm stress and don't know what to do. Any advice for me?