I can say this from 2 perspectives- I was anorexic in high school- didn't have my period for 14 months and experienced lots of health problems. Now, 11 years later I am slightly overweight and am trying to be healthy without getting back into that bad place in my mind. People of all shapes and sizes can be beautiful, in fact all ARE, but many cover it up.
I am a high school teacher (I see beautiful girls getting breast implants, nose jobs, being promiscuous or dressing in a certain overly skimpy way to fit in, etc.) and a beauty advisor with Avon (I do makeovers and teach people how to be their most beautiful)
Personally, I hate seeing someone in something truly unflattering because I know how much better they could look (this goes for someone 100 lbs or 400 lbs) if they wore a different color or style bathing suit.
I also get upset seeing such young girls trying to show an overtly sexual image to the world because in my experience there are psychological and societal issues that that child has. When I see a 14 year old girl in a string/almost thong bikini, pregnant, trying to act "cool", I just want to go over to her, put a big sweatshirt on her and hug her and tell her she doesn't have to act that way- that her body and soul are beautiful and she should respect them, show the world she respects them and demand that respect from others.
I've never done this, btw, nor would I, just expressing the emotions I feel at seeing something like that.
As I said, I see nothing wrong with wearing a bikini while pregnant as long as it is flattering to the woman and she is comfortable.