The female is expected to assume child rearing duties, and is judged an individual success on the basis of her motherhood role. When we look at a large number of societies we can see a pattern when it comes to fertility level. There are societies where the social system is structured in such a way so as to induce high fertility levels. In these societies the family is still the primary unit of economic production. Children are great assets for parents.
High fertility is encouraged.
In modern industrial societies there is widespread individualism. In these societies couples are encouraged to have children - parenthood being associated with reaching full maturity, etc. However, large numbers of offspring are not encouraged. Children are a source of emotional satisfaction with great psychic rewards, but these rewards can be attain with one or two children, five or six are not needed. That it declined to four pregnancies by 1900 can also be easily understood - the 19th century saw the US industrialize and urbanize.