Actually you are quite wrong here. I am a trained social scientist and know a bit about this. If a country increases in prosperity, the birthrate drops. A high birth rate is a product of poverty. It is not true that a high birth rate causes poverty, the reverse is true.
The working person prosperity of the UK is largely a post-war phenomenon. I have one brother, my mother had two brothers, but her mother had 6 sisters and a brother, and my mother's father had 6 brothers and one sister. This is a phenomenon true all over the world. Poverty decreases, birthrate drops - Poverty increases, birthrate rises