The reason in Africa region most people give birth to many children is  because they lack the knowledge of birth control, in most Africa countries they do not encourage people to give birth to the number of kids they can afford to take care of. One of the things that can reduce the number of population is if the government encourages parents to have the understanding of birth control, this will really help in decreasing the population in a short time.
In developed world that population is very low they are able to control their population because birth control is something everyone is aware of and know how to apply it.