Fertility rates decline most strikingly due to the education of women in society:
http://www.earth-policy.org/data_highlights/2011/highlights1...
"Female education is especially important. Research consistently shows that women who are empowered through education tend to have fewer children and have them later. If and when they do become mothers, they tend to be healthier and raise healthier children, who then also stay in school longer. They earn more money with which to support their families, and contribute more to their communities’ economic growth. Indeed, educating girls can transform whole communities."
A bit older study:
http://www.cdc.gov/nchs/pressroom/97facts/edu2birt.htm
"A women's educational level is the best predictor of how many children she will have, according to a new study from the National Center for Health Statistics, Centers for Disease Control and Prevention. The study, based on an analysis of 1994 birth certificates, found a direct relationship between years of education and birth rates, with the highest birth rates among women with the lowest educational attainment.
Birth rate patterns also vary greatly by mothers age. Among women in their twenties - the peak childbearing ages - and women in their forties, birth rates are highest for women with the least education. For women with college degrees, rates are highest for those in their early thirties, perhaps signaling the preferred time for childbearing by this group. First birth rates for women in their thirties with a college degree were two to five times the first birth rates for women with less education."