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.