since reliable birth control has been available to women. Before that, births were a natural result of being an adult. It was not uncommon for women (even educated women) to have 4 or 5 kids.
Considering the cost of housing and the wacky schedules that people work , even people who WANT kids sometimes cannot have as many as they would like.
Costs are a major cause too. If you are a responsible person, but you have 40K in college debt, it's gonna take you a LONG time to pay enough down to be in a place where you can afford a child (and the loss of a job?) By the time you are ready, fertility may be an issue . My youngest and his wife married after they were 35, and they were unsuccessful at pregnancy, so they are childless and are satisfied with being Aunt & Uncle..They would have been super parents, but it was not to be 
The US workforce is not child-friendly. I had 3 boys within 5 years and even though I wanted to work when they were younger, daycare would have eaten up most of what I made, so I stayed home until they were all in school all day.
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