Women awarded doctorates in science and engineering made up a vast proportion of the increase in the number of PhDs bestowed in the US last year.
The number of doctorates awarded by US universities rose by 1.6 per cent between 2008 and 2009, according to the National Science Foundation's (NSF) annual Survey of Earned Doctorates.
The total, now nearly 50,000, represents a 20.6 per cent rise on 1999 levels.
The number of non-science PhDs awarded rose by 1 per cent in 2009, to 16,086, while the number of science and engineering doctorates rose by 1.9 per cent to 33,470.
The latter was accounted for entirely by a 4.8 per cent rise in the number of women earning science PhDs. This figure now stands at 13,600, a rise of 38 per cent since 2004, compared with a 21 per cent increase for men (now almost 20,000).
The number of women awarded PhDs in non-science subjects has also risen by more than 3 per cent since 2004, compared with a decline of almost 1 per cent among men.
Total numbers of non-science PhDs awarded in 2009 were 9,597 for women and 6,489 for men.
Meanwhile, the proportion of doctorates being awarded to non-white Americans is also growing thanks to increases of 34 per cent and 16 per cent since 2004 in the number of black and ethnic minority scholars earning doctorates in science and the humanities respectively.
For white candidates, the same period saw the number of PhDs awarded increase by 22 per cent in the sciences and fall 0.9 per cent in the humanities.
Stephen Joel Trachtenberg, president emeritus of George Washington University, said the figures reflected the growth in participation among women and ethnic minorities at undergraduate levels and would probably continue to rise.
He said that the trends reflected the "quiet revolution" that meant white people would soon become a minority in the US.
"One may anticipate one day seeing language in job postings encouraging applications from white male candidates who are underrepresented in academic posts," he said.
The overall rise in doctoral numbers was also being driven by increasing demand from people of Asian origin, particularly women, Professor Trachtenberg added.
"Professional and doctoral degrees are a ladder out of the working class: that is a classic in American sociology for first-generation families," he said.
The NSF figures record a rise in the number of doctorates awarded in most subjects last year, with the largest increase recorded in mathematics.
Computer science doctorates fell by nearly 10 per cent year on year, but still posted a rise of nearly 90 per cent since 1999.
Doctoral numbers fell by 5.4 per cent in psychology over the decade, the only science discipline to record a drop. In the humanities, numbers declined 7.3 per cent over the period.
The number of PhD recipients with specific job offers fell slightly last year to 70 per cent, with the lowest rates recorded in the humanities and life sciences.