A report released Thursday by the federal government revealed that U.S. birth rates declined in 2017 for women in their teens, 20s and 30s, leading to the lowest number of newborns in 30 years.

According to experts, several factors may be contributing to the declines, including changing attitudes about motherhood and shifting immigration patterns.

The provisional report was compiled following a review of more than 99 percent of the birth certificates filed nationwide last year, and calculated 3.853 million births — the lowest number since 1987.

Although the number of births has been declining since 2014, the greatest year-to-year decrease occurred in 2017, with about 92,000 fewer births recorded than in the previous year.

Experts were surprised by last year’s decline since baby booms often parallel economic booms, and 2017 saw low unemployment rates and a growing economy.

Other factors are likely taking a toll on birth rates, experts believe.

“One may be shifting attitudes about motherhood among millennials, who are in their prime child-bearing years right now. They may be more inclined to put off child-bearing or have fewer children,” USA Today reported.

Experts are also pointing to changes in the immigrant population, who give birth to nearly a quarter of the babies born in the U.S. annually. For example, Asians currently represent a larger proportion of immigrants, and they typically have fewer children than other immigrant groups.

Another contributing factor could be the increased use of IUDs and other long-acting forms of contraception.

The report from the Centers for Disease Control and Prevention also found:

  • The rate of births to women ages 15 to 44, known as the general fertility rate, dropped to a record low of about 60 per 1,000.
  • Women in their early 40s were the only group with higher birth rates in 2017, up 2 percent from the previous year. The rate of birth among women in their 40s has been rising since the early 1980s.
  • The cesarean section rate rose by a small amount after having decreased for four years. Studies have shown C-sections are more common in first-time births involving older mothers.
  • Rates of preterm and low birth weight babies rose for the third consecutive year.
  • Birth rates for teens continued to decline, as they have since the early 1990s. In 2017, they fell seven percent from the year before.
  • Rates for women in their 20s continued to drop and hit record lows.
  • Birth rates for women in their 30s declined slightly, dipping 2 percent for women ages 30 to 34 and 1 percent for women 35 to 39.

The report also noted that the current generation is getting further away from having enough children to replace itself. Ten years ago, the estimated rate was 2.1 children per U.S. woman. In 2017, it dipped below 1.8, marking its lowest level since 1978.

“That’s a pretty remarkable decline,” said Dr. John Santelli, a Columbia University professor of population and family health and pediatrics.