In my contract with the nhs they can make me take mat leave early if I fall sick within four weeks of my due date
If you take any sick days after week 36 they can force start of maternity leave in any job. Maternity pay is calculated up to week 25 so any sickness now won't effect it
I don't think it affects maternity pay but legally they can force you to start your leave earlier if you're off due to pregnancy related illness