I am 5'5", 175ish lbs @ 32w1d pregnant. I was 154lbs prepregnany. I bought a medium.
Size-wise, I would say to buy whatever size you would pre-preg and you should be fine.
I bought this dress for a wedding. I guess it depends on your bust and personal preference, but I feel that this dress is very boob-y. A little too much for me but my husband said it was fine (go figure). This isn't the best picture but its the only one I have. The straps were definitely too long. I was going to sew them but I ended up just tying a knot in the back of each one and it was fine. I think that the dress was flattering, since the band is right below the bust, which is where most women are smallest. I used one of those sticky bras and that worked well. What I don't like about this dress is the material. While the fabric is very soft and I was comfortable the whole evening, it is very thin. I wouldn't have dared going braless. Its not see-thru per se, but any little shape is visible. This is true for the backside especially. I wore (Belevation brand, which I recommend btw) shapewear shorts underneath.