This is our policy. https://www.doncasterpct.nhs.uk/documents/MaternityLeaveandMaternityPayPolicy.pdf
If an employee is ill during pregnancy before they begin Maternity Pay/Leave, the illness may or may not be associated with the pregnancy, the employee will, on production of a sick note, be entitled to receive sickness benefits in the usual manner.
If the illness is associated with pregnancy and occurs before the fourth week prior to the expected week of confinement, they will be entitled to receive sickness benefits in the usual manner. If the sickness continues beyond the fourth week prior to the expected week of confinement, Maternity Pay/Leave will start automatically at that time. The employee will thereafter be treated in accordance with the maternity regulations.