But a proper healthcare system should absolutely be discussing obesity with women who intend to get pregnant at least, just as it should be discussing it with people who want to have surgery requiring general anaesthesia, with people who have mobility issues or knee/hip problems, etc.; if not more generally even outside of the context of immediate interventions.

Because it makes difficult, traumatic and bad health situations worse. Giving birth is routinely the first two of those things.

There is a positive-discrimination aspect here, though: yes, obesity complicates the health of older people, is raising mortality rates across the board. But society doesn't want women to die in childbirth anymore, and that makes this targeted intervention re: obesity a proper thing to do.