Where an employee takes annual leave prior to her maternity leave, but gives birth early, will her maternity leave still start after the period of annual leave?

No. Ordinary maternity leave is automatically triggered by childbirth and will start on the day following childbirth, even if it is premature. Given that it is not possible for employees to take annual leave and maternity leave simultaneously, if an employee gives birth while she is on annual leave, her annual leave will cease and her maternity leave will commence.