Make sure to check your contract re mat leave, I'm conscious in some work places they have parameters e.g. how long you've worked there for in order to provide paid maternity leave. I would write to your line manager to ask about the probation, as typically until you're signed off you wouldn't officially have passed it. Have you had a final probation meeting?
I would reach out to management to say something like, my probation period has passed can I assume I am now out of probation? Or something like that. Push them on it so you've got something clear in writing.
If your probation date is gone then you are past your probation and protected accordingly. If you tell your employer you are pregnant, you benefit from the rights and protection associated with it.