Dr. William Doherty discusses class and family structure in the U.S. Middle and upper class people tend to delay marriage and child bearing and get married before they have children. Also, the divorce rate in these classes has gone done over the last 15 years.
In lower classes, child bearing occurs before marriage and very complex family structures result because of parents having children by a number of different partners.
The video lasts 8:34 minutes. It is worth watching if you are interested in the topic.