I think its absolutely fine to wear one while pregnant, no matter how big. I mean I've seen girls on the beach who are "fat" not pregnant but "fat" with rolls everywhere, wearing two piece or other skimpy bathing suits, now that's NOT acceptable.
I'd go for it.
I'm working on my sixth kid right now.
Due in May.
the one before this one, was due july 15th.
i wore a bikini around july 10th.

perfectly fine.