The main reason for the population explosion in China's early years is, in short, the birth rate mismatch caused by low medical conditions and rapid improvement of medical conditions.
In the case of low medical level, if the food supply can support it, the fertility strategy adopted by human society is to have more children. This is because under such conditions, due to diseases, natural disasters and other reasons, the probability of death of children and adults is very high. If you have fewer children, you may have no children and daughters, resulting in no one to take care of the elderly and no one to inherit the family business. The usual birth size is (if other conditions permit) five or six or more.
With the improvement of medical conditions, the survival rate and life expectancy of children have been greatly improved. At this time, the old birth strategy will naturally lead to rapid population growth.
With the long-term improvement of medical conditions, people will revise their fertility strategies according to this new normal, so the fertility rate will gradually decline.
If the improvement of medical conditions is a slow process, this is naturally not a big problem. However, if the medical conditions improve rapidly and the average life expectancy increases by leaps and bounds, as in China after 1949, then it will naturally be impossible to revise the old birth strategy synchronously (it will take at least one generation, because only one generation can produce a new round of experience summary on birth strategy), and then there will be a big population explosion in a generation or so (20-30 years).
This situation is consistent with the change of fertility rate in China after 49 years (except during the famine). Of course, there are also factors such as the gradual popularization of contraception, the gradual popularization of education, the rising status of women, and the general participation of women in social work. This is just one of the main reasons.
In addition, 1949' s "400 million population" is an estimate of the population during the Republic of China, but due to the war, this statistical data is incomplete. According to the later census, it may exceed 500 million people in 49 years.
As for the future development, Zhihu already has too many good answers about the current situation and future of the population problem, so it's not embarrassing here.