My workplace gives us 13 weeks paid maternity leave, 6 of which is meant to be taken before the due date (ie. from 34 weeks for a 40 week due date). If we want to work longer (and thereby maximise the leave after the baby), we need a doctors certificate saying we are fit to work. So in effect we can take sick leave before the 34 week date but after that if we're not fit to work, we'd start going into maternity leave in any case because the doctor couldn't give us a sick leave certificate AND a certificate saying we're fit to work.

Similarly after the baby's born, we can take up to 12 months unpaid leave (once the 13 weeks has run out) and if we're sick during that time, we take sick leave which then extends the unpaid leave. But we'd need a certificate and baby-related 'conditions' such as "breastfeeding" would not qualify as sickness.

Hope that makes some sort of sense!

So I would think that you can only use sick leave if you're actually sick and have a doctors certificate.