I informed the company of my pregnancy at 3mths. So I held off my IVF until at least 6 months into the job, at least I would have made some contributions and proven myself before I may have to tell them they need to start looking for maternity cover. (IVF is just 60% chance but I didn't want to risk it, in the end I was lucky to be first time successful). I think you have to weigh your risks and rewards Eg how likely is it you will have to announce during probation, what the company culture is like, worst case scenario how easy it is to find a job or to conceive after the 6 months window.
Some company have regulation that during probation can’t get pregnant. If get pregnant will ask to leave. My best friend encounter before. She tell me she was not read this T&C. If not she will wait. End up pregnancy period she only can work temp job until she deliver. After that she only can rest 1 month and fast fast find a perm job.