Can getting scanned by metal detector while 20+ weeks pregnant hurt baby?

No: Radiation exposure should be minimized during pregnancy. However, low level casual exposure as a metal detector are not significantly powerful enough to harm.

Very unlikely: The security detectors use a low-frequency electromagnetic field to look for weapons. This frequency is safe for everyone, even pregnant women. These are not Xray Machines. But if you don;t feel comfortable you always can ask the officers to pat you down instead.