New fathers in the United States are getting older.
Researchers at Stanford University reviewed data on 168,867,480 live births from 1972 to 2015, making statistical adjustments for missing paternal records. The average age of the father of a newborn in the United States, the investigators found, has risen to 30.9 from 27.4 in 1972.
Paternal age increased across the country: the oldest fathers lived in the Northeast, and the youngest in the South. There were average age increases across all educational levels, races and ethnicities. The report appears in the journal Human Reproduction.
In 2015, fathers with college degrees were 33.3 years old on average, compared with 29.2 for those with only a high school diploma. Asian fathers were the oldest on average by ethnicity, and blacks and Hispanics the youngest.
Japanese fathers saw the largest increase in average age during the study period, from 30.7 in 1972 to 36.3 in 2015. White fathers were on average 27.6 years old in 1972 and 31.1 in 2015, while the average age of black fathers increased to 30.4 from 27.2.
Over the study period, the percentage of fathers older than 40 increased to 8.9 percent from 4.1 percent of all annual births, the percentage over 45 to 2.9 percent from 1.5 percent, and the percentage over 50 to 0.9 percent from 0.5 percent.
The trend is not exclusive to the United States. In Germany, for example, the median age of fathers rose during the 1990s to 33.1 from 31.3. In England, fathers over 35 accounted for 40 percent of all births in 2003, compared with 25 percent in 1993.
"One thing that did surprise me is that the difference between mothers' and fathers' ages has been decreasing," said Dr. Michael L. Eisenberg, the lead author of the report and director of male reproductive medicine and surgery at Stanford.
"The father is still older than the mother, but the gap is narrowing."
Older paternal age has been associated with higher rates of miscarriage, birth defects, some cancers, schizophrenia and autism. Some experts have suggested that older sperm is more likely to have mutations that lead to disease.
But a study last year in Nature Genetics concluded that such mutations are probably a small part of the cause. Instead, men who are genetically predisposed to psychiatric and other illnesses are also more likely to delay fatherhood, and those genetic tendencies are inherited by their children. The question remains unsettled.
A little more than 10 percent of birth certificates don't include paternal data, a problem that needs fixing, Dr. Eisenberg said. "Fertility is a team sport. It's a disservice to ignore one half of the equation."
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