The birthrate in America has been declining, but some places are more fertile than others, according to a new look at federal data that reveals significant variation in fertility rates around the country. Only South Dakota’s and Utah’s fertility rates reached the level needed to sustain the current population.

The number of babies born in 2017, around 3.85 million, was the lowest since 1987. In order for the country’s population to essentially replace itself, researchers say that 2,100 babies should be born for every 1,000...