The Ergo might last you a good while if your waist is not too big to start with. But the strap and buckle do go around the waist, so personally I haven't even tried mine while pregnant. It seems like it would be uncomfortable.
For something to wear on the back (which is most comfortable for me now) or the front, I am finding a mei tai to be the most comfortable. Although a lot of the time we just use a ring sling or adjustable pouch in the hip carry.