If you’re off sick during your qualifying weeks then it might affect your maternity pay if you don’t earn enough. If you’re off sick in the last 4 weeks of pregnancy with a pregnancy related illness your work *can* enforce your maternity leave to start, doesn’t mean they will though.
I was going to say this too- depends on your work. I know for me it was from about week 30 that any sick leave counted as maternity leave. Before then not. Could you maybe go off with stress rather than pregnancy induced illness? Might give you some extra security re maternity pay.