(CNN) -- The number of births in the United States declined for a fourth year following an increase in 2014, according to a new report from the US Centers for Disease Control and Prevention's National Center for Health Statistics.
The report, which analyzed the 2018 birth rates and fertility rates in the United States, also confirmed that the US fertility rate continued to decline last year. The 2018 general fertility rate hit a "record low," the researchers wrote in the report, published Wednesday.
The new report found that the number of births among girls and women age 15 to 44 registered in the United States last year was just under 3.8 million, down 2% from 2017 and marking the fourth year that the number of births has declined since 2014.
The total fertility rate estimates the number of births that a hypothetical group of 1,000 women would have over their lifetimes, based on age-specific birth rates in a given year.
The total fertility rate declined to 1,729.5 births per 1,000 women in 2018, also down 2% from 2017 and marking the fourth year that the total fertility rate has declined following an increase in 2014, according to the report.
In 2018, the total fertility rate for the United States remained below what's needed for the population to replace itself, according to a separate report published by the National Center for Health Statistics in January.
America's fertility rate and the number of births nationwide have been on the decline in recent years. A report of provisional birth data published by the National Center for Health Statistics in May showed the number of births last year dropping to its lowest level in about three decades.
Now the center's latest report presents selected highlights from that 2018 birth data.
For the report, researchers examined birth certificate data from the National Vital Statistics System's Natality Data File, taking a close look at births among white, black, Hispanic and Asian women in 2018.
When examined by race, the data showed that fertility rates declined 2% for white and black women, 3% for Hispanic women and 4% for Asian women between 2017 and 2018.
The data also showed that the teenage birth rate -- for girls ages 15 to 19 -- fell 7% from 2017 to 2018. When examined by race, the data showed that teen births declined by 4% for black teen, 8% for white teens, 10% for Hispanic teens and 15% for Asian teens.
Also among all births, the percentage delivered at less than full term, or 39 weeks, increased -- with preterm births climbing from 9.93% of births in 2017 to 10.02% in 2018, and early-term births rising from 26% in 2017 to 26.53% in 2018.
Rates of preterm birth rose most significantly among black women -- from 13.93% in 2017 to 14.13% in 2018; and for Hispanic women, from 9.62% in 2017 to 9.73% in 2018.
The percentages of births delivered at full-, late- and post-term declined, according to the data. Full-term births were down from 57.49% of births in 2017 to 57.24% in 2018, the data showed, and post-term births declined as well.
Of the women who gave birth in 2018, there was a 6% decline in those who smoked during some point of their pregnancy. Younger women under age 30 were more than twice as likely to smoke during pregnancy than women over 40.
Overall, a rise in preterm births might be linked with a rise in births among women in their late 30s and 40s, since a later maternal age is a risk factor, Dr. Rahul Gupta, chief medical and health officer for March of Dimes, a nonprofit focused on the health of mothers and babies, said about last year's report, which showed the same trends. He was not involved in either report.
"The continuing shift toward increased maternal age at first birth is something that does increase the risk. However, it does not fully explain the increase in the preterm birth rate. So that's one of the challenges here, I think, for the nation," he said. "There is a lot more work that needs to be done as the preterm birth rate continues to rise."