Most mothers find that their relatives or midwives offer advice that warns against wearing underwire bras during nursing. Their advice is usually based on accounts they have heard from other people, not on established medical findings. The warnings stem from the idea that the bra wire impedes circulation and flow in the breasts. This is supposedly the cause of too little milk, lumps, or even mastitis. There are no proven links between lactation problems and underwire bras.
Your own comfort should be your guide. Don't wear an underwire bra if it feels uncomfortable. The reason most nursing bras don't have wire is due to the comfort factor. If you choose to wear an underwire bra and experience lactation problems, try going for a few days with a softer nursing bra. If the problems go away then avoid underwire bras. Not only will you find that nursing bras are more comfortable, but also more practical when breastfeeding. If you have large breasts, it is fine to wear a more supportive bra when you feel the need to.