Hi. I was high risk in my last pregnancy (twins). I was due 27th of January. I planned to start my leave on the 6th of January when we returned after Christmas break. I knew I could alter it to start sooner if needed. At 29 weeks, my consultant signed me off work. I was huge, in pain with PGP, and generally struggling. I was signed off at the beginning of November time. My sick leave would have taken my until just before New Year, and then my mat leave would have triggered as I would have hit 36 weeks.
My girls decided to come on Christmas Eve, 35 weeks. My mat leave started the day after birth (technically).
I understand your question. And no. If you are on sick leave for a pregnancy related illness, they will trigger your maternity to begin in the 36th week. If you give birth in the 36th week, they can not back the maternity to start 4 weeks earlier again.
Also, for you to be aware as you've said you are high risk, new legislation has come in for parents of premature babies in regards to additional fully paid leave.
On another note, please look into shared parental leave as a teacher. It can massively boost your income.
Finally, if you plan to return to work September 2026, you can officially return on the first day of the summer holidays and receive fully pay for the 6 weeks. You DO NOT need to go in to work in July to do this☺️ congratulations, I hope all goes smoothly.