Their biological clocks may be ticking, but fewer U.S. women are listening—at least fewer young women.

The country’s birthrate fell to a record low in 2016 with 62 births per 1,000 women of childbearing age, but that top-line figure conceals a main driver of the decline: The birthrate among teenagers fell 9% from the previous year to 20.3 births per 1,000 women, the lowest figure for that age group since at least 1940.

The...