I would wait. If you're on probation, they can fire you without a reason. Don't give them the incentive to use their advantage.
They can do that anyway at any time. You don't have to have a reason to fire an employee beyond "They are not a good fit for our team".
That said- wait. It's not unusual for people to not tell their work early on unless necessary. It's a personal choice. As long as you aren't waiting until you are 35 weeks to say "So hey, I'm going to need a couple months off in about a month or so" you should be fine.