Why households in poor countries have more children? Is it because of religious reasons, i.e., no contraceptives allowed. Or is it because of disease, i.e., high infant mortality rates due to disease result in parents having a large number of children because the parents are not certain how many children, if at all, will survive.
The following animation presents data from the World Bank that suggests that with a decline in infant mortality rate over time, the fertility rates declined as well. What could also be extrapolated is that with the decline in the incidence of numerous diseases, the infant mortality rates have declined sharply in certain countries. The lag with fertility rates implies that it would take some time for the fertility rates to decline in response to the lower infant mortality rates.
Each bubble in the animation represents a country.