What factor is most likely responsible for the spike in europe's population that took place from about 1800 to 1950?

In the nineteenth century several nations that underwent rapid industrialization and urbanization also experienced high population growth rates. The acceleration of population growth in the nineteenth century was a direct consequence of declining death rates and stable or even rising fertility rates. In England rising birthrates produced much of the growth, and these were, in turn, the consequence of increased incidence of marriage and earlier age at marriage and not of rising marital fertility.