You can start maternity leave any time from 11 weeks before the beginning of the week your baby's due. You can bring the date forward if you need to take time off work for pregnancy-related reasons within four weeks of the expected birth date.Your employer may make you start your leave from then.o claim your right to maternity leave, you must tell your employer about your pregnancy no later than 15 weeks before your baby is due. You can work this out by finding the Sunday before your baby is due, and then counting back 15 weeks. You should tell your employer: Your employer may want you to put this in writing.