This piece on what drives trends in U.S. birthrates looks at shifts toward older ages at first births for American women as a potential cause. Lisa Neidert says it’s important to consider average age at childbirth by region. She says that while the most common age to have a first child in the northeastern states is 30-34, in Oklahoma and Arkansas women typically have their first child when they are in their early 20s. Starting younger increases the likelihood of having a bigger family, which is why the fertility rate is significantly higher in the West than in the northeast.

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