In the second trimester of my first pregnancy, and these are the best leggings I've purchased.
Size guide is 100%, go with your pre-pregnancy size and you will be comfortable.
Unlike other brands, these don't roll down (as long as you get your correct size), and they are so comfortable that I basically wear them all day!!
I'm usually a size 10, and the Small was a perfect size around my hips and bump. Also purchased a Medium, and unfortunately these were *just* too big, and I did experience some rolling down of the waistband.