I've never had this problem and I really don't know why others do?

I feed my bitch good, keep her healthy before during and after pregnancy and when I wean the pups I do it gradually. Even bred an almost 10 year old bitch who completely dried up, no saggy titties. You can't even tell the bitches have been bred, only difference is slightly larger nipples.

So while I didn't offer you any advice, by my own experience of just giving good care I've never had this issue.

S_B