The most common construction for a push-up bra is an underwire design. Underwires are sewn into the base of the bra and run under each cup. They can be made of molded plastic, flexible wire or plastic-coated metal. Modern innovations have made underwires much more comfortable to wear while still providing great support. Some have underwire between 2 layers of fabric. Others are constructed of a very flexible plastic which keeps its form without becoming misshapen or breaking.
Underwires provide crucial support, especially in a bra meant to shape and lift, like push up bras. A push-up underwire bra will incorporate both the underwire and graduated padding in the lower part of the cup. Some are also constructed with side boning and/or a rigid fabric for additional support.