As has already been said it is really about your breasts changing shape quickly during pregnancy. Underwire is fine as long as your bra fits properly, but they often don't when you are pregnant. I would have had to buy about 4 different lots of bras if I wanted to stay in underwires the whole time.
In my experience, if you don't feel supported in a maternity bra then you are wearing the wrong size. There are plenty of fabulously supportive bras, but they generally are the most expensive ones. I have no choice but to get the most expensive ones as my breasts are large. I am in a 10G at the moment and it hold my boobs up perfectly well.
In fact I had a great deal of trouble going back to underwire bras after I stopped feeding DS2, and went on a great search for a wirefree bra in a 10E (not easy to find). Got a couple in the end.