Just found this on a UK website which I thought might be helpful!!!!
To give the correct notice, you must let your employer know about your pregnancy by the 15th week before the week that you are expecting your baby (called the Notification Week or the Qualification Week). Work this out by going to the Sunday before your due date (or staying with that date if your baby is due on a Sunday, and counting back another 15 weeks.
During the notification week, you must tell your employer:
that you are pregnant and intend to go on maternity leave
that you want to receive Statutory Maternity Pay (SMP)
the planned date that you will be starting your maternity leave (which cannot be earlier than 11 weeks before your baby is due).
Then your employer will reply within 28 days, stating the date that the company expects you to return if you take your full leave entitlement.
If you need to change the date, you will have to give 28 days' notice.