American men are delaying fatherhood. New research has found that the average age of newborns' fathers in the US has grown by 3.5 years over the past four decades.

Between 1972 and 2015, the average paternal age at the time of an American child's birth grew from 27.4 years to 30.9 years, showed the findings published in the journal Human Reproduction.At upwards of 36 years of age on average, Asian-American dads are the oldest, the study showed. The steadily advancing age of newborns' fathers is likely to carry public health implications, said senior study author Michael Eisenberg, Assistant Professor at Stanford University School of Medicine. A rising paternal age can affect the total number of children a man will have, which can impact the demographics of the population. 

"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," he said. The research showed that paternal age rose with more years of education. The typical newborn's father with a college degree is 33.3 years old.Men over the age of 40 now account for about nine per cent of all US births. Men over the age of 50 account for nearly one per cent.The findings are based on an analysis of all live births reported to a federal data depository in the US from 1972-2015. The data included 168,867,480 births.The Stanford researchers obtained the data from the National Vital Statistics System, an intergovernmental data-sharing programme sponsored by the federal Centers for Disease Control and Prevention.

"Maternal ages at birth have been increasing, too," Eisenberg said."The average age difference between moms and dads has been shrinking, from 2.7 years in 1972 to 2.3 years in 2015," Eisenberg added.