Can an employee change her mind about when she wants her maternity leave to start?

Yes, provided that she notifies the employer of the revised start date at least 28 days before the earlier of the new date and the original date. If it is not reasonably practicable for her to do so, she must provide this notification as soon as is reasonably practicable. The employer can request that notification of any revised date be provided in writing.

The date that she selects to start her maternity leave cannot be before the start of the 11th week before the expected week of childbirth.