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Average U.S. newborn’s dad is getting older (as is mom) — and it matters

The average age of a newborn's dad has crept steadily upwards in the United States, from 27.4 years old in 1972 to 30.9 in 2015. Three and a half years may not sound like a lot, but to demographers that's a huge jump over a mere four decades or so.

Men over the age of 40 now account for about 9 percent -- and men over the age of 50 for nearly 1 percent -- of all U.S. births. (I helped beef up the first statistic once upon a time.)

These numbers come from the most comprehensive analysis of paternal characteristics ever recorded for the United States, encompassing virtually all live births in the U.S. from 1972 through 2015 -- to be precise, 168,867,480 births in all. The study, published in Human Reproduction, was led by Stanford urologist and big-data cruncher Michael Eisenberg, MD.

The advance in paternal age is a double-edged phenomenon, Eisenberg told me. From my news release on the study:

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, [Eisenberg] 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.

Moms are getting older, too, by the way. (Maternal data has been more readily available over the decades.) In fact, Eisenberg told me, the average age of a newborns' mother has advanced even more quickly than has the average father's. This could be a consequence of women waiting longer to get married or, aided by increasingly widespread and more-reliable contraception, putting off childbearing as the years they spend in higher education increase and as careers become more central to their lives.

It follows that couples giving birth are not as far apart in ages as they used to be. From 2.7 years in 1972, the average age difference between moms and dads shrank to 2.3 years in 2015. This convergent pattern appears to apply to all racial, regional, age and education categories.

Advancing parental age leaves fewer years for childbearing and, most likely, reduces average family size over the long haul, with potentially huge social and economic ramifications.

Of course, there will always be outliers. From my release:

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 this 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.

Previously: Male infertility can be warning of hypertension, Stanford study finds,  Poor semen quality linked to heightened mortality rate in men, Low sperm count can mean increased cancer risk and Men with kids are at lower risk of dying from cardiovascular disease than their childless counterparts
Photo by Karen Sheets de Gracia

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