Underwire bras are a type of brassiere that have a thin, flexible wire running along the bottom of each cup. The wire helps to give support and shape to a person's breasts. Full definition
It is sometimes suggested that pregnant women shouldn't wear underwire bras as the wiring can sometimes cause blockages in the milk ducts. (mindbodygreen.com)
It's best to avoid underwire bras while breastfeeding since they can contribute to worsening of engorgement. (checkpregnancy.com)
Also, a reminder - there are many, many studies and research that are starting to connect underwire bras with breast cancer - the wires block the lymph nodes from doing its job (draining toxins). (drmomma.org)