Oh shame!

There's always at least one person that has to upset you.
I agree that you should really just shut her up next time she starts. Say something like "My doctor says it's all fine." and then walk off.
I've luckily not had any know-it-all people here at work. Only one extremely rude one. She enjoys pointing at my stomach and sniggering then calling me fatty. I crapped her out yesterday for not watching where she was going and nearly knocking me over while I had a plate of food in my hand. She avoided me ever since.
The only know-it-alls I've had are those who have never even been pregnant. I try to keep my distance really lol because my temper is horrible these days I'm going to end up offending them.