In most cases if the underwire of the bra is small and thin, it can go through the metal detector, like how zippers on pants can go through. If the bra size is larger, and the underwire is thicker and larger, then it has a higher chance of setting off the metal detector.

Yes, airport metal detectors can occasionally detect bras, but it primarily depends on the type of bra and the sensitivity of the machine. Most traditional bras contain underwires made of galvanized steel or nickel, which can trigger the alarm if the metal content is high enough. This is more common with older "walk-through" metal detectors. However, modern security screening has shifted toward Millimeter Wave Scanners (the circular booths where you raise your hands). These scanners do not use magnetism to detect metal; instead, they create a 3D image of the body's surface to look for concealed objects. In these machines, an underwire might show up as an "anomaly" on the screen, leading to a quick, targeted pat-down by a female security officer. To avoid this inconvenience, many frequent flyers choose to wear "travel-friendly" bras, such as sports bras, wireless bras, or those with plastic underwires. If you do trigger the alarm, it is a very routine occurrence for security staff, and they will typically use a handheld wand or a brief manual check to verify the source of the metal is simply your undergarment.