Although they gave birth to more children before the industrial revolution, most died, so the population was almost constant. If we define "birth rate" as the number of children that survive long enough to have their own children, then I believe birth rate was close to 2 back then. I believe long term the birth rate (according to my definition) always converges to 2 because anything else is not sustainable. Nothing can exponentially increase forever. Maybe the population can exponentially decrease to 0 but it's unlikely because there always be some group that will be willing to have more children for cultural or religious reasons.