I would wait until probation ends if that will be by 15 or 16 weeks.
It is illegal to discriminate but why risk it? If you tell them now and then fail probation then you'll never really know whether it was because of your pregnancy or performance. If you keep it to yourself, then you know you have passed or failed on your own merits.
It would also be entirely normal to jeep this news to yourself until then. A lot of people tell no one until after the 12 week scan, and then will want to tell friends and family before work. So it could easily be 15 weeks before you're ready to tell your employer.
With my first pregnancy, I didn't tell work until 16 weeks because of end of year appraisals. I had been working for my very fair employer for 7+ years, but I wanted to be sure my appraisal was fair. There is no possibility that the pregnancy influenced my bosses judgement because he didn't know! He was also not annoyed by the delay, and in fact it was another month before we told my team (which was tricky to hide as I was carrying twins!).
I am now pregnant again (13+ weeks) and won't be telling my boss for a few weeks yet if I can keep my stomach sucked in. We're just setting objectives for the coming year, and I don't want all the good stuff going to someone else because I'll be going on leave. Once its been assigned to me, I can then work up handover plans, but why should I risk getting stuck with the dross because I'm pregnant?!
If you need significant time off due to pregnancy, then I would consider telling them earlier - it'd be crazy to fail probation because of pregnancy related sick absence! But if you only need the odd day/half day for the booking appointment and first scan, then I'd take them as annual leave and bide your time.
good luck with your pregnancy, your new job, and your announcement when you tell them. I hope it all goes well :)