Are you always accidentally offending the larger type of woman by asking when the baby's due? This can easily be avoided by carrying around a tiny baby.
When she says "I'm not pregnant." simply bend down, pretend to pick up the baby (which will be concealed in your hand or sleeve) and say "What's this then?"
The baby will have to be very small just in case she's wearing trousers and you need her to believe that it could have fallen through the leg. Hand her the baby then run away.
Awkward situation dodged!