If your normal bras are starting to feel a little uncomfortable, it's time to go for a fitting. If you have not done so already, get yourself measured professionally. Wearing a good and supportive bra during pregnancy is essential to prevent backaches and sagging breasts.
Whenever you feel you need a new bra, get measured properly to ensure that you're wearing the right size. Although your breasts may be growing very quickly at this time, you should find that by the end of the first trimester the growth has stabilized. There is then unlikely to be much more breast growth until you're in your third trimester and after the baby is born.
Wearing underwire bras during pregnancy is not recommended because they can dig into the developing breast tissue and damage it, and may even cause problems with milk production; the wires digging into the skin can also be uncomfortable. Non-underwire bras with wide supportive straps, such as sports bras, are good in pregnancy.