Newly trained female doctors in the United States make nearly $17,000 less than their male counterparts, even though women increasingly are choosing careers in higher-paying medical specialties, U.S. researchers said on Thursday.

They said there has been a widening gender gap in starting salaries for female doctors, rising from a difference of $3,600 in 1999 to $16,819 in 2008.

"It is not surprising to say that women physicians make less than male physicians because women traditionally choose lower-paying jobs in primary care fields or they choose to work fewer hours," Anthony Lo Sasso of the School of Public Health of the University of Illinois at Chicago said in a statement.

"What is surprising is that even when we account for specialty and hours and other factors, we see this growing unexplained gap in starting salary. The same gap exists for women in primary care as it does in specialty fields," said Lo Sasso, whose findings were published in the journal Health Affairs.

The study is based on survey data from more than 8,000 doctors exiting training programs in New York, a state that is home to more residency programs and resident physicians than any other in the United States.

While historically women have tended to choose primary care fields such as family medicine or pediatrics, the percentage of women entering those fields dropped from about 50 percent in 1999 to just over 30 percent in 2008, roughly on par with male doctors.

But even though women doctors are choosing higher-paying medical specialties, they still made considerably less than men in 2008. The team found the gap widened even after adjusting for choice of specialty, practice type or number of hours worked.

"Essentially, what we see is our ability to explain that difference in salary between men and women goes away. What we're left with is almost a $17,000 salary difference between men and women," Lo Sasso said in a telephone interview.

"We weren't expecting to see this widening gap in salary over time and the inability to account for it with any observable characteristics."

While the study does not show why, Lo Sasso said the team cannot rule out gender bias as an explanation. But he thought it likely that female doctors were taking less pay in exchange for regular schedules or other family-friendly benefits.

"What we think it is essentially women trading off some salary for other non-monetary aspects of the job," he said.