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. She wasn't a huge rebel either. She always dressed nicely (my grandmother didn't agree, though, She thought Jeans are not an appropriate attire to go shopping in and offered my mum to make her some nice ruffled blouses and pleated skirts).
Seriously, to me, this sounds like 1950s attitude. Did your mother miss the late 60s and 70s???

And for the record, I see beautiful heavily pregnant women in bikinis at the beach all the time. I talked to one and asked her how far along she was and she said: "I was due yesterday". lol

In my opinion, anyone should be allowed to wear what they're comfortable in. If an old overweight man can wear budgie smugglers, then why can't a pregnant woman wear a bikini? Not that I'm comparing the 2.

Your mum probably thinks I shouldn't wear one either. I'm a size 16.
One question: Is your mum wearing a bikini herself?