The (total) fertility rate of a population is the average number of child births per woman. The sub-replacement fertility rate is usually at 2.1 births per woman; below this rate populations will not grow through reproduction.
Developed countries usually have a much lower fertility rate due to higher living standards and the individualism culture. Mortality rates are low, birth control is easily accessible, and children often become an economic drain caused by education costs, clothing and feeding. In developing countries on the other hand, children are a necessity as labour in fields and as carers and providers of their parents in old age. Fertility rates are also higher due to the lack of access to contraceptives.