From the abstract: > Using data for 1.7 million first births from 1990 U.S. natality and mortality records, we compared outcomes for women with first births (p

From the abstract: > Using data for 1.7 million first births from 1990 U.S. natality and mortality records, we compared outcomes for women with first births (primiparas) aged 16–20 years (when first births typically occur in forager and subsistence groups) with those aged 21–25 years. The younger primiparas had a much lower risk of potentially life-threatening complications of labor and delivery So a reasonable assumption is that they mean 16-20 years.