Some physical conditions make it painful or difficult for women to wear bras with underwires. If you are pregnant, disabled, or have recently had surgery for breast cancer, you may prefer a wire-free bra. But it can be tough to find one with adequate support and structure, especially for large busts.
Wired vs. wire-free bras fit somewhat differently, because wire-free bras don’t have the structure of the wire for lift and support.
What that means for women is that they need the part of the bra that connects the cups (the gore) to be taller, so there’s more ability for the cup to offer the lift women love. Taller gores mean more coverage, which women don’t always love - so there’s often a trade off. Plunge bras, in order to work best, generally need to have some wire for structure.
In addition to a taller center gore, look for at least three rows of hooks where the bra closes to help anchor the band. Side boning or stays can also make for a more secure fit, if you can tolerate them (they tend to be somewhat more comfortable to wear than an underwire).
The Lara wire-free bra from Amoena has a substantial center gore, plus side stays and three rows of hooks. It’s a full-coverage style, making it a good fit for a wide range of bust sizes. But what really distinguishes it from similar bras that offer great support without underwires is that it's attractive, available in a variety of colors, and the smooth cups are invisible under clothing. This is one of my favorite wire-free options on the market right now (yes, I own one and I wear it regularly).