Horseshoe, I've copied this for you from the DTI website:
An employee cannot normally start her maternity leave unless she has given her employer the required notice, except in the following circumstances:
If the employee gives birth before the date she has notified, or before she has notified a date, her maternity leave period starts automatically on the day after the date of the birth. She must notify her employer as soon as is reasonably practicable of the date of birth.
If the employee is absent from work due to a pregnancy-related reason after the beginning of the 4th week before the expected week of childbirth but before the date she has notified, her maternity leave period begins automatically on the day after the first day of her absence. She must notify her employer that she is absent from work wholly or partly because of pregnancy and of the date on which her absence for that reason began as soon as is reasonably practicable.
MORE:
Absence for a pregnancy-related reason before the intended start date
An employee who is absent from work due to illness will normally be able to take sick leave until she starts maternity leave on the date notified to her employer as described above. If the illness is unrelated to her pregnancy she can remain on sick leave and receive Statutory Sick Pay/Incapacity Benefit right up to the date of the baby?s birth, or until the date she has notified as the date on which she intends her maternity leave to start.
If, however, the illness is pregnancy-related, the maternity leave period starts automatically on the day after the first day of absence following the beginning of the fourth week before the expected week of childbirth. This applies even if the day of absence is before the date the employee has notified as the date on which she intends her leave to start. In order to preserve her rights, the employee must as soon as is reasonably practicable give her employer notice (in writing if requested) that she is absent from work wholly or partly because of pregnancy and of the date on which absence for that reason began.
Odd days of pregnancy-related illness may be disregarded at the employer's discretion if the employee wishes to defer the start of her maternity leave period.
The same arrangements apply if the employee?s absence is for some other pregnancy-related reason. However, time off for attending ante-natal appointments in the four weeks before the expected week of childbirth will not trigger the start of leave (and the employee has the right to time off work to attend ante-natal appointments throughout her pregnancy).
Pregnancy-related absences should not be used for disciplinary purposes, in redundancy decisions, or for any other detrimental purpose as this would be unlawful sex discrimination.
You can read more about Maternity Rights on:
www.dti.gov.uk/er/individual/mat-pl958.htm
Hope that helps.