Over the same time period, the share of newborns’ fathers who were older than 40 doubled from 4.1 percent to 8.9 percent, while the share who were over 50 rose from 0.5 percent to 0.9 percent.

Similar trends of increasing age have been reported in other industrialized countries.

The steadily advancing age of newborns’ fathers is likely to carry public-health implications as well, Eisenberg said. A rising paternal age can affect the total number of children a man will have, which can impact the demographics of the population. In addition, he said, “every potential dad acquires an average of two new mutations in his sperm each year. And there are associations between older fatherhood and higher rates of autism, schizophrenia, chromosomal abnormalities, some pediatric cancers and certain rare genetic conditions.”

On the flip side, he noted, older fathers are more likely to have better jobs and more resources, more likely to have reasonably stable lifestyles and more likely to live with their children and, thus, be more involved in child-rearing.

Ages of new moms increasing, too

“Maternal ages at birth have been increasing, too,” Eisenberg said. “In fact, they’ve advanced even more than paternal ages have in the same time frame. This may be a consequence of women waiting longer to get married or putting off childbearing as the years they spend in higher education increase and as careers become more central to their lives. The result is that the average age difference between moms and dads has been shrinking, from 2.7 years in 1972 to 2.3 years in 2015.”

This convergent pattern appears to apply to all racial, regional, age and education categories, he said. “We’ve seen a lot of changes in the last several decades. Contraception is more reliable and widespread. Women have become more integrated into the workforce. This seems to be reflected in an increasing parity in parental ages over the last four decades.”

Fewer people being born means fewer productive workers a generation down the road.

Advancing parental age leaves fewer years for childbearing and is likely to exert a follow-on effect of reducing the average family size over the long haul, with potentially huge economic and public-health ramifications, Eisenberg said.

“Fewer people being born means fewer productive workers a generation down the road,” he said. “This can obviously have profound tax and economic implications.”

While parental-data reporting to the National Vital Statistics System by some states was spotty in the early years of the period under study, it’s been running at virtually 100 percent since 1985, at least for mothers, said Eisenberg. In 2015, the latest year available, information about newborns’ fathers was missing in one of every nine births. That could be because the father was unknown or because the mother didn’t wish to report his name or any details about him, Eisenberg said.

It matters. Evidence indicates that, on the whole, young children whose paternal data appears in their birth records have better health outcomes.

Paternal-data reporting rates vary according to mothers’ race, ethnicity, age, education and regional location, the analysis showed, with the reporting rate for African-American newborns consistently the lowest for all races. Over the past decade, African-American mothers under the age of 20 reported paternal data only half the time. However, the rate of paternal reporting for children born to African-American mothers has grown from its low of 63 percent in 1985 to a current rate of 70.9 percent, Eisenberg said.

Overall paternal reporting for U.S. births has risen to its current 88.4 percent since reaching a nadir of 85.5 percent in 1991.

The youngest dad recorded during the 44-year period covered in the study was 11 years old; the oldest was 88. But the world-record holder, Eisenberg said, is a gentleman from India who early in the last decade fathered two children at the age of 94 and 96 with a wife who was in her late 50s. “Unfortunately, they wound up separating,” he said.

Eisenberg is a member of Stanford Bio-X, the Stanford Child Health Research Institute and the Stanford Cancer Institute.

Other Stanford co-authors of the study are biostatician Chiyuan “Amy” Zhang, MPH, and Ying Lu, PhD, professor of biomedical data science.

Stanford’s Department of Urology supported the work.