Ok, I've come back to read the replies and can't believe how different Australia used to be to Europe. You're only 24, so your mum was pregnant with you 7 years after my mum was pregnant with me. And my mum didn't bat an eyelid at donning a bikini in 1979. She was not the only one, either.
Seriously, to me, this sounds like 1950s attitude. Did your mother miss the late 60s and 70s???
I don't think it is about missing the 60s and 70s. By Europe I assume you mean mainland Europe and not England. We Aussies have always been a bit prudish in many ways like the English, and unlike other Europeans.
My mother's explanation to not liking bikinis on pregnant women was that she didn't find the really stretched looking skin on a very pregnant belly attractive, and in a way I can understand that. It wasn't about the shape of a pg belly or that you should hide being pregnant.
It doesn't make me stop wearing a bikini though. I don't like the look of a lot of people in their swimwear, but it doesn't stop them either.