The earliest that you can start your maternity leave is 11 weeks before your expected week of childbirth, which is when you are about 29 weeks pregnant. To ensure that you get all your rights, you should use the due date given on your MAT B1 pregnancy certificate that your midwife or GP will give you.
In order to work out when the 11th week before your expected week of childbirth falls, you should find the Sunday before the date which the certificate states your baby is due, or the due date if it is a Sunday, and count back eleven Sundays from that date. The earliest date on which you can start your leave is that Sunday. As long as it does not fall before the 11 weeks before your baby is due, it is generally up to you when your maternity leave begins.
You can even choose to work right up to the date the baby is born. It is for you to decide when your leave will begin.
You should note that the start of your maternity leave might be triggered automatically if:
Having been notified of your intention to take maternity leave and the date on which it will begin, your employer must give you notice of the date that your maternity leave period shall end. Your employer must provide you with this information within 28 days of you informing them of the date that your leave period will start on or has started on.