OK, so immediate spaying is out. I didn't know if that was the case.

I felt that, at the very least, the pet being in heat would be a good reminder that she needed to be spayed, and calling the vet to set that up would be a good idea.

Of course, if you've never had a cat or dog in heat before, it would fall into the "you notice a significant change in behavior" category. It's been decades since I've been around cats in heat, but it might also fall into the "your pet smell funny" category.