The birthrate in California has plunged yet again, to its lowest level in a century, according to the latest U.S. statistics, as young adults continue to postpone having or not have families.

Fewer than 12 children were born per 1,000 California residents in 2017, according to preliminary figures released Thursday by the Centers for Disease Control and Prevention. With a population of 39.5 million, there were 471,552 babies born in California last year, for a birthrate of 11.9 per 1,000 people. That’s about half what it was in 1990.

Nationwide, 3.8 million babies were born in the U.S. in 2017, a decline of 2 percent from 2016. That was the lowest number in 30 years, the CDCsaid.

Birthrates nationwide declined for women under 40 but rose slightly for women in their early 40s.

Causes of the decline, according to population experts, included young people deciding to put off parenthood for financial or personal reasons, along with increasing availability of birth control and the state of the U.S. economy.

Women, said UC Berkeley demography professor Josh Goldstein, have more control over their decisions biologically but perhaps less control over their decisions economically.

Goldstein said birthrates fluctuate as the percentage of women of childbearing years varies within the population. He said the latest decline was “not something to worry about.”

Nationwide, the fertility rate declined 3 percent from 2016, to 60.2 births per 1,000 U.S. women ages 15 to 44.

Steve Rubenstein is a San Francisco Chronicle staff writer. Email: srubenstein@sfchronicle.com