Mat leave

I’ve previously spoken to my manager about going on mat leave early (12 wks before due date) and they are about to put the request up to fill my position. However, my doctor has recommended sick leave until I’m due. If I go on stiip after they have put the request in for August, does that mean my mat leave will still start in August? Or will it be the 6 weeks before my due date like the BCGEU agreement has.