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Maternity leave of 14 continuous weeks may start up to six weeks before the expected date of birth.
In certain cases maternity leave can start earlier. Your doctor or midwife can direct you to start your leave earlier if they believe it is necessary for the health of you or your baby. Your employer can also direct you to start maternity leave early if you cannot continue to do your job safely or cannot perform your job adequately. If your doctor or employer directs you to start your maternity leave early, you have the right to take eight weeks maternity leave after the expected date of birth, and your maternity leave is extended to allow that. Special rules apply in this case - contact the Department of Labour on 0800 20 90 20 during business hours for further details.
If you and your employer agree, you can start maternity leave at any other time before your baby is due.
ERS website