system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do a girl's breasts first develop during puberty, and not during a woman's pregnancy? Would it be too much stress on the body to develop breasts while pregnant?

Curious, because there's not much of a point needing/having breasts before having a baby.
gpt
It's called a "genuine signal" like birdsong or bright plumage, breasts can't be faked. If a female has breasts she is likely able to reproduce. So if you're an early human, it's a very easy way to see if a female is worth pursuing as a mate.