Just found this on the internet.
"There does not seem to be any concrete evidence to show that wearing underwired bras during pregnancy may be harmful. However, there appears to be a potential risk. Many midwives and retail outlets advise against purchasing underwiring in pregnancy. The reasoning behind this advice appears to be that the rigid wire may interfere with the natural changes in size and shape of the breast during pregnancy. This may obstruct the increased blood flow or "squash" of the developing milk duct system, causing pain, discomfort and possibly mastitis."
So i think that if your not in pain or any extra discomfort after wearing your underwired bra i wouldn't worry about it.