I definitely agree that it's a fairly recent thing that pregnant bellies are celebrated rather than hidden. I think it's very fashionable to wear a bikini whilst pregnant. It's very fashionable to be pregnant, LOL! I am of course of the opinion that bellies are lovely, as is breastfeeding, and wouldn't bat an eyelid at either being displayed in public (unless it was to get a bit reminiscent and sad that I won't be in that stage of life again).
I do understand though how you'd get cross with your mum, particularly if she was apologising or talking about you with her friends That would upset me more than her opinion on what I was wearing.