A recent report said the U.S. birth rate has dipped to a record low level. But another measure of the nation's fertility remains comfortably above its historic low. The mismatch shows that even in a country with comprehensive birth statistics, summarizing population trends is far from straightforward.

Last week, Pew Research Center said the birth rate last year fell to 63.2 per 1,000 women age 15 to 44. That's the lowest level since at least 1920, the earliest year for which reliable data are available. The report made headlines...