Some pregnant women choose to go with a regular bikini because they don't have to worry about the middle part getting in the way. But as the day draws closer, you will probably want a one-piece. You should pick one that you can let out and adjust as your stomach and hips expand. You will probably want some support in the bust as well.
Avoid buying a cheap swimsuit. You will be better off with one or two higher quality swimsuits that are sympathetic to your condition and will last through the season.
Resources:
WWW.HIGHBEAM.COM
WWW.DEEPBLUESWIM.COM
WWW.SUNCITYINC.COM