Many things have contributed to the birth rate. One of them is the culture of the area you look into. for example places that teach abstinence/ protection and contraceptive methods may have a lower birth rate than places that have little to none. Another thing could be areas have limits on birth by law. Like in highly populated areas they will limit family's to one child, unless they are under special circumstances and given permission by the government, namely china. other thing that effects it is the mood of the society, like after ww2 we have the baby boom because people coming home were so happy and eager to start a family.