I would wait so they cant refuse or if your confident that they will take you anyways let them know. Either way if they say your hired they can not fire you or change their mind because of your pregnancy
Keep in mind that a workplace is required to give you mat leave after 20 weeks. And they can in fact fire you if they find out before your probation. That's why there's a probationary period, so they can fire you on the spot without notice. I'd wait and see if you even get hired, and then pop in to HR and see what they think. HR is usually super nice. And you still have 6 months, not 5 (; babymaking takes 10 months, not 9 as they go by your last period.