For reference I usually wear a small in tops, I have a petite hourglass frame, busty, small waist. I ordered the small,  but I'm sure the x-small would've worked well also. It has a slightly loose fit to it and I can see it stretching if needed for those asking about a maternity fit. My only issue with the top is that the sleeves are a tad long. Hate when that happens. I do not have skinny arms, but perhaps my shorter height has something to do with it? Maybe if you're 5'6 or taller the arms would fit more appropriately? I'm just concerned with the extra 1-2 inches in the sleeves that it would look bad once wet /swimming.
Pricey, but it's just what I was looking for as far as a modest swim top is concerned.