The metal detector and body scans are different things. The metal detector is just sensitive to...metal.. is an old technology that has been around forever and is perfectly safe. And you probably can't opt out of it unless you have a metal/electrical implant like a pacemaker and a doctor's note.
While they say the new body scan tech (where you stand in that tube/box and raise your arms) is safe, it hasn't been around very long so the longer term effect on pregnant women can't have been exhaustively studied. While it's probably fine, I'd opt for a pat down on this one, since that is usually an option anyway.