I think that is not true at all. We're made pretty tough. And if the baby can go through the bumps (not too much though!) they will grow to be a stronger individual.only if you're pregnant or having a serious cramp should you worry about doing too much exercises. Talk to a doctor, or maybe google a bit if you still have questions unanswered.
I'm guessing old wives tale. Although if u are currently TTC, I'd avoid heavy lifting. I exercise, am 25 and have 4 children...