The number of U.S. women taking maternity leave has not changed in 22 years despite factors that suggest it should be increasing, a new study found.

During the same time, the number of fathers taking paternity leave more than tripled, although the numbers are much smaller than those of women taking time off.

The research showed that about 273,000 women in the United States took maternity leave on average each month between 1994 and 2015, with no trend upwards or downwards. Fewer than half of them were paid during their leave.

But during that time the U.S. economy grew 66 percent and three states (California, New Jersey and Rhode Island) enacted and implemented paid family leave legislation.

"Given the growing economy and the new state laws, I expected to see an increasing number of women taking maternity leave," said Jay Zagorsky, author of the study and research scientist at The Ohio State University's Center for Human Resource Research. "It was surprising and troubling that I didn't.

"There's a lot of research that shows the benefits of allowing parents, especially mothers, to spend time with newborn children. Unfortunately, the number of women who receive those benefits has stagnated."

Meanwhile, the number of men taking paternity leave increased from 5,800 men per month in 1994 to 22,000 per month in 2015.

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The research was published Jan. 19 in the American Journal of Public Health. Zagorsky used data from the Current Population Survey, a monthly survey conducted by the U.S. Census Bureau, for the study.

Since 1993, most workers have been covered by the federal government's Family and Medical Leave Act, which gives eligible employees 12 weeks of unpaid time off during the first 12 months after birth to care for a newborn.

Zagorsky found that most women who took maternity leave were not paid -- only 47.5 percent were compensated in 2015. Paid maternity leave is increasing, but only by 0.26 percentage points per year.

"At that rate, it will take about another decade before even half of U.S. women going on leave will get paid time off," he said.

"This is a very low figure for the nation with the world's largest annual gross domestic product."

In comparison, 70.7 percent of men on leave were paid for their time off in 2015.

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"One possible reason for this gender gap is that few men are willing to take unpaid leave to care for a newborn," Zagorsky said.

Women who took maternity leave were economically better off than the typical mother -- as described in the data -- and were more likely to be married, white and more educated, study results showed.

"The Current Population Survey, which is the underlying information source for the study, provides a wonderful resource for tracking family leave. If the survey asked about recent births it would be possible to determine exactly the percentage of working parents who ever use this valuable benefit," he said.

Zagorsky said the best estimates from the data would be that somewhere around 10 percent of men and 40 percent of women take some time off.

Zagorsky said that based on the data he reviewed, the lack of increase in the number of women using maternity leave cannot be blamed on trends in the number of working women or women dropping out of the work force.

The stagnation is particularly surprising because the United States experienced dramatic economic growth during the time period of this study, with the inflation-adjusted gross domestic product rising from $9.9 trillion a year in 1994 to $16.4 trillion in 2015.

"It would have been reasonable to expect that some of the benefits of this large economic expansion would have gone to working women with newborn children, but that's not what I found," he said.

It was also surprising that the new paid family leave laws in California, New Jersey and Rhode Island didn't have an effect on maternity leave numbers, Zagorsky said. Those three states in 2015 included 16.1 percent of the nation's female labor force.

"If the laws were effective, some impact should be seen in national data," he said.

During the presidential campaign, President-elect Donald Trump promised to provide six weeks of paid leave to new mothers. But as of 2007, the United States was one of only four countries (out of 173) that lacked paid leave for women after childbirth.

"These results suggest we have a long way to go to catch up with the rest of the world as far as providing for new mothers and their children," Zagorsky said.