I was offered a job when I was 10 weeks pregnant. I told my employer to be, because I felt it was only fair to let them know as soon as possible so they would know how long they had with me. To have withdrawn the job offer would have been discriminatory, and pretty bloody obvious. Besides, they got to use my skills for 6 months, and for that they were grateful.
I know this isn't the case with all employers, but if you've been offered the job in the first place, then they obviously think you're worth employing. To sack you within 3 months would be just as much upheaval for them as finding maternity cover in 6 months time. If you started the job whilst pregnant then your maternity leave won't cost them a penny. You'll get SMP direct from the government. This could be something to mention when you do tell them you're pregnant.
Today's my last day of working and my employers have been totally supportive of my pregnancy. They've allowed me to work from home 2 days a week for the last month, take as much time off as necessary for ante-natal appointments.
I can understand your reluctance to want to say anything until you're completely sure this pregnancy is safe, but I think once you've reached that point you'll want them on your side. And that's more likely to happen if you're up front and honest. It'll be a huge weight off your shoulders. Why not let them deal with the burden of finding a replacement for when you come to take your maternity leave, rather than you deal with the stress of covering up your pregnancy?
However, if you'd rather wait till your probation is over then you're not obliged to say anything till 24 weeks.