Sounds like _A Farewell to Alms_ by Clark. Though as far as I can remember the data showed the "merchant class" crowding out the lower classes, not the nobility. Nobles only had about an average amount of surviving children while rich merchants had an above average one. Makes more sense that way: medieval nobility doesn't sound like a great base for middle-class values...

I'm not totally convinced about the theory, but the book was a surprisingly good read.