Actually I am a jogger and I kept active in my pregnancy - walking, swimming gently
But this is a far cry from running 26 miles on a hard road when birth is imminent. No I don't think women should lie on the couch all pregnancy but there is a balance. I don't buy the idea that your body can do everything it did before. But then this is symptomology of the culture we live in - even pregnancy is competitive. There was a study that showed heavy exercise, particularly in the first trimester increases the risk of miscarriage. Why take the chance?
Yes they have to walk long distances in third world countries. But they also have a higher rate of birth trauma, fistula damage, maternal death etc. Not a good comparison