and generally you can't take leave more than 6 weeks before the birth (the legal term is date of confinement)
Ellibugs,

Where I work it states that the co. can request you to stake leave up to 12wks prior to your EDD (or date of confinement) say if they thought my role jeopardised my pregnancy etc & there wasnt another role for me to do...

It also says we can start leave at any time during our pregnancy, but must notify them of any medical conditions that could harm myself or the baby etc... So as to cover their butts!!