I did tell my boss before I started as even though my bloods had already been and would continue to be before I normally start work, I wasn't sure how many tries I would have and didn't want to have to worry about lying to my boss for the retrieval and transfer dates. I was worried about all the hormones turing me into a crazy woman too (which they didn't really) I am autonomous at work so I could easily have just been late or taken afternoon off, but I felt much better after telling my boss.
I told my boss I had been trying for a long time already so it was very realistic that IVF would take a long time too. I also said I trusted my boss not to share it with anyone , including HR, as it was not going to impact my work - in particular though because I hadn't even told my family as I found people knowing made it more stressful for me.
Most people these days know someone who has infertility problems so I think if you are upfront it is much better as people are sympathetic if they know what is going on.
Best of luck to you