I'm not as optimistic as Chollie about your chances in UK airports.

The consensus here is that most people get the WTMD and are on their way, but if you're "chosen" for the nude-o-scope, it's either be scanned or don't fly. I don't believe there's an official exemption for pregnant women, but you may get a decent security agent who gives her a break. Or not.

(Paging stifle!) I'm not sure whether MAN still has the x-ray scanner; it's not clear what the risk level is and I would not want a pregnant relative to be scanned with x-ray.