Which is regarded as the defining year to mark the demographic transition from its first to the second decisive stage?

1921 is regarded as the defining year to mark the demographic transition from its first to the second decisive stage. It is also called the year of "great divide" because before 1921 the population growth in India was stagnant and it had high birth rate as well as high death rate but after 1921 India's population growth never declined and has always shown upward trend.