Researchers for the Centers for Disease Control and Prevention (CDC) reported Thursday that the national total fertility rate (TFR), an estimate of how many babies the average woman will have, was 1.7655 in 2017. That’s down from 1.8205 in 2016 and 1.8435 in 2015.

This matters because the population needs to have a TFR of 2,100 births per 1,000 women to reproduce itself. If the U.S. wants a stable population, it either needs more babies or more immigration.

Digging into the results by state reveals that Americans vary in their procreative habits, with a low TFR of 1.4210 in the District of Columbia and a high of 2.2275 in South Dakota.

Hispanic women in the U.S. continue to have a higher TFR than other Americans, averaging 2.0065 nationwide, and with a wider variation between states, from a low of 1.2005 in Vermont to a high of 3.0850 in Alabama.

But neither Hispanics nor other Americans have high enough TFRs to replace the population without immigration. Two states are exceptions: South Dakota and Utah.

The trend is global: overall TFR has declined steadily since 1960, reaching a new low of 2.439 in 2016.

While today’s report offers no explanation for the drop, a CDC researcher commenting on a related report earlier this year said fewer teen pregnancies are partly responsible. A July New York Times survey also suggested that young adults are postponing having children until later in life because of financial worries.

The TFR is also sensitive to the age at which women have children. So if a population starts having children later — as is happening in the U.S. and most rich countries — the TFR may underestimate their future fertility.

“In general women are getting married later in life,” public health researcher John Rowe at Columbia University told NBC News, “They are leaving the home and launching their families later.”

In contrast, the TFR may also over-estimate Hispanic fertility, because recent migrants tend to have higher fertility rates soon after arrival in the U.S. than they do once after they settle in.

Not everybody worries about declining populations, however: some economists argued in Science in 2014 that shrinking populations lead to increased standards of living, though the researchers acknowledged that, “children yield direct satisfaction and impose costs on parents that are difficult or impossible to measure.”