To be fair, the biggest challenge before you is getting the size right. Too small and she’ll think she’s fat. Too big and she’ll think that you think she’s fat... and you’re (back) in the dog box.
Best place to start is to sneak a peek in her knicker drawer... like you haven’t before. Now the secret here, other than not getting caught, by pretty much anyone, is to pick a bra that she is wearing currently and that fits her well and that she’s not already bursting out of. This may require you to pay more attention, conducting even more painstaking research, like checking her out next time she’s wandering about the house in her smalls... like you haven’t before.
So back in the knicker drawer check out the size tags. With bra sizes the number is her rib-cage circumference or 'under-bust' and the letter represents her breast or ‘cup’ size. Now bear in mind that when a woman gets in the motherly way she usually will go up at least one cup size from what she normally is. (Yeh, nice one!)
Now make note of the sizes. And write it down - yeah you know you need to.
Look, if you’re getting really stuck, just grab one or two, shove ’em in your back pocket and head on down to the store. Discreetly of course.
Or you could also enlist inside help by requesting a close female friend’s consultation. Choose your ally carefully though. They can be crap at keeping secrets from each other.