I think a lot has to do with the density of the breasts and the skill of the technician, the skill of the radiologist, etc....in my case, I had a 2.5 cm tumor that was palpable but not visible on the mammogram, since I was not quite 31. Now at 36 I am hoping that my mammos are more reliable, as my breast tissue changes. Who ever thought I'd be praying to lose density? The ultrasound also showed my tumor clearly.
I asked my surgeon about the reliability of mammos for me, and he said just because one cancer did not show up, does not mean any future problems would not be visible either...each stands on its own.
I think whatever is the first mammo that they can get a read on would be your baseline.
Just my thoughts...