I am 5'5", 150lbs, and 14w pregnant and I ordered a large based on the reviews. I normally order a small or medium in undies. I was worried that they would be too large when I opened the package, but I think they just looked large because they are full-bottomed. They ended up fitting really well. Full bottom, but low in the front to allow for the belly. Im happy to have undies that don't roll down over on themselves because of my growing belly!
The fabric is a nice medium weight with good elasticity and very comfortable. I will likely be buying more.