Probably the biggest of the lot – most other problems stem from it.
I have studied this in depths. There are cultural and religious obstacles to overcome.
What is required is:
a. Female education (a lot of 3rd World countries do not educate girls) – educated girls have less children.
b. Education in general – to educate men as well so that they understand and can progress.
c. Contraception availability
d. Medical nurses availability to explain and care.
e. A welfare system that provides when someone is ill, or in need – so one in not dependent on children.
f. A pension system that means that the old are not dependent on their children.
g. A campaign to explain to people (the current Indian one is proving quite effective – You me and two + family)
It is not by chance that the fertility rates in developed countries is much less.