I would meet with her now before fertility treatments really ramp up. The time off will inevitably increase. I would tell her exactly what you wrote in this post. Your schedule changes are starting to effect our careers. We were very flexible because you are valued, but we are starting to feel the pushback and we can't keep changing things for non-emergent issues.

She'll either realize that she was about to ask for an extremely flexible schedule for blood draws at 7am and ultrasounds and think about quitting early or she will make it work and stop asking for the random schedule changes. She keeps doing it because you are happily saying, "yes".

These hours should be paid sick time, PTO, or unpaid, just as my employer would expect of me. I could see having the conversation “could I stay later two days this week and leave at 3:30 on Friday?” But only if your work is flexible and you’re willing.

Why are you double-paying guaranteed hours for time she asks for off? If she wants time off that you need her working, then that should reduce her guaranteed hours or come out of her PTO. Beyond that, this doesn't sound workable long-term...since even if you take care of the pay issues, it's hard to have an unreliable nanny. You need to talk to her, and let her know this isn't working for you.