The number of births for the United States last year dropped to its lowest in about three decades, according to provisional data in a new report from the National Center for Health Statistics at the US Centers for Disease Control and Prevention.

"Even though the number of births we've seen in 2018 is the lowest that we've seen in 32 years, the total fertility rate is at a record low," said Brady Hamilton, a natality expert at the center and first author of the report.

The report, published Wednesday, showed that birth rates declined for nearly all age groups of women younger than 35 but rose for women in their late 30s and early 40s.

From 2017 to 2018, the birth rate dropped 7% among teenagers aged 15 to 19; 4% among women 20 to 24; 3% among women 25 to 29; and 1% among women 30 to 34, according to the report.

The birth rate rose 1% among women aged 35 to 39 and 2% among women 40 to 44. The rate for women 45 to 49, which also includes births to women 50 and older, did not change from 2017 to 2018.

Overall, the provisional number of births in 2018 for the United States was about 3.79 million, down 2% from the total in 2017, according to the report. The finding marks the fourth year that the number of births has declined, after an increase in 2014.

A falling fertility rate

The new report was based on birth certificate data from 2018, processed by the National Center for Health Statistics. Although it was able to show trends in birth rates, it cannot say why they have occurred.

"These data provide the official statistics on birth for the United States," Hamilton said. "The data allow you to monitor patterns, in terms of birth-related health issues for infants and mothers, such as cesarean delivery, preterm or low birth weight rates."

The provisional data -- which will be finalized later in the year -- showed that the total fertility rate for the United States last year was 1,728 births per 1,000 women, a decrease of 2% from 2017 and a record low for the nation.

While birth rate compares the average number of births during a year per 1,000 people in a population, total fertility rate is defined as the expected number of births that a group of 1,000 women would have in their lifetimes, according to what birth rates were by age for that year.

The total fertility rate in 2018 was below what is considered the level needed for a population to replace itself: 2,100 births per 1,000 women, according to the report.

"The rate has generally been below replacement since 1971 and consistently below replacement for the last decade," the authors wrote in the new report.

The total fertility rate for the United States in 2017 was 1,765.5 per 1,000 women.

The data also showed that the preterm birth rate in the United States rose for the fourth year in a row to 10.02%, from 9.93% in 2017.

That rise was due to an increase in late preterm births, in which a child is born at 34 to 36 weeks gestation; early preterm births, in which a child is born at less than 34 weeks, have declined slightly, according to the report.

The rise in the preterm birth rate might be linked with the rise in births among women in their late 30s and early 40s, since a later maternal age is a risk factor, said Dr. Rahul Gupta, chief medical and health officer for March of Dimes , a nonprofit focused on the health of mothers and babies. He was not involved in the new 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."

Preterm birth, or premature birth, is when a baby is born too early. The earlier a baby is born, the higher the risk of death or disability, so reducing preterm birth is a national public health priority , according to the CDC.

'There is still reason for concern'

From a public health perspective, the data showed some improvements in first trimester prenatal care and cesarean section births, said Dr. Georges Benjamin, executive director of the American Public Health Association , who was not involved in the new report.

"One of the more interesting statistics was the fact that cesarean sections were down," he said. The overall cesarean delivery rate decreased from 32% in 2017 to 31.9% last year in the United States.

An overuse of elective cesarean section, or c-section , has become of concern because the procedure can pose maternal risks, including infection or postpartum heavy bleeding.

C-section is a surgical procedure used to deliver a baby by making an opening in the mother's lower abdomen area. Although the procedure can be planned, many occur when life-threatening childbirth problems arise.

The report also showed that 77.5% of women received first trimester prenatal care in 2018, up from 77.3% in 2017. That rise indicates possible improvements in overall access to health care.

"That was good, to see that percentage of women getting first trimester prenatal care is up," Benjamin said, adding that as the Trump administration explores changes to health care that create barriers to insurance coverage, this trend might be affected.

"We look at the whole range of maternal-child health care as a great barometer of access to care, because there has been an emphasis over many years of making sure that women and children -- even low-income women and children -- get access to health care," he said. "That's through the original welfare programs and through Medicaid and through many states' health care programs and the federal children's health insurance program. So there's been a national effort to make sure that women get the care that they need."

Notably, the data showed an increase in early prenatal care among both black and white women, said Dr. Heather Burris, an attending neonatologist at Children's Hospital of Philadelphia and an assistant professor at the University of Pennsylvania Perelman School of Medicine , who was not involved in the new report.

Yet "there is still reason for concern," she said, because a large disparity persists between black and white women's receipt of early prenatal care, according to the report.

The data showed that last year, 67.1% of black women received first trimester prenatal care, compared with 82.5% of white women, 81.8% of Asian women and 72.7% of Hispanic women.

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Meanwhile, the percentage of women receiving early prenatal care dropped between 2017 and 2018 among American Indian, Alaska Native and Native Hawaiian or other Pacific Islander women, according to the report.

The report also showed racial disparities in preterm birth rates, Burris pointed out. The overall preterm birth rate in the United States rose from 9.93% to 10.02%, according to the report, and disparities in that rate remain.

"Black infants are more than 50% more likely to be preterm and more than 200% as likely to be born low birth weight than white infants," Burris said. "These ongoing disparities are of grave concern because they are largely responsible for the large black-white disparity in infant mortality."