How to get people to give up their seats to pregnant women

Those signs over the "courtesy" seats on public transports - "Please give up this seat to someone who needs it more than you" are just not working.

An alternative sign:

"If I'm sitting in this seat, I'm either elderly, disabled, pregnant or just an inconsiderate bastard and a disgrace to my family. And, yes, I'm only pretending to sleep/read this book/newspaper."