If the piercings are small enough than there should be no problem. If the piercings are metallic, a bit larger and many then they could cause problems at the Metal Detector.
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Although some have claimed otherwise, there are thousands that wear underwire bras through airport security each day without setting off the alarm. If you really want to avoid setting off the metal detectors at all costs, consider wearing an underwire bra with plastic, a strapless bra or a sports bra.
Metal jewelry and piercings can set off the metal detector. Likewise, belt buckles can easily cause issues, and most TSA rules and regulations require you to remove them anyway.
I know this is a boring answer, but more than likely nothing is going to happen. Most jewelry doesn't contain enough metal & is too small to trip detectors (and I say this as a person who's had up to a dozen piercings at one time and never once had a problem walking through a metal detector).