Usually when the maternity leave is paid there are different rules to the regular 52 weeks unpaid leave that everyone is entitled to.

I'm currently on paid maternity leave and with my employer the leave can't start any earlier than 10 weeks before your EDD. If you want to go on leave earlier than this you can use your paid holiday leave or unpaid leave.

You'd probably be best to check with your personnel dept as to what the conditions are at your workplace.