More women than ever are opting out of having kids, according to new data from the U.S. Census Bureau. Nearly half of women between the ages of 15 and 44 didn't have kids in 2014, and that's the highest number since the government started measuring that statistic, Time reports.

Birth control advances and reform may play the biggest role in the decline. In the 1970s, when the government first started tracking fertility statistics, the Pill had only been around for a decade. Now, it's widely used, which means women can choose when they want to get pregnant. Fewer teenagers than ever are having babies, thanks to expanded access to contraception.

But some of the urge to go child-free might also come from career pressures — women from 40 to 50 were more likely to not have kids if they were in managerial or professional occupations. And Census statistics showed that women who don't have a child by 40 are unlikely to have one later. (Note: Women over 44 aren't officially counted in the general fertility rate, since births at that age are still rare, but they are on the rise.)

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People are also choosing to get married later and delay childbirth, which has lowered the divorce rate but has also lowered the birth rate. In fact, the birth rate in the United States dropped for the sixth consecutive year last year, to 1.86 babies born per woman. That's under the 2.1 rate that's necessary to keep the population stable, even though the number of women in key childbearing ages has risen recently.

The economy may also be to blame for the low birth rate, since it's easier to afford one child (or none) than two ore more. According to the Census stats, women who did have children were more likely to stop at one; the number of women from 40 to 44 who only had one child doubled between 1976 and 2014.

But there's no need to stress out, says family demographer Andrew J. Cherlin. "Americans haven't worried much about birthrates in the past, because we have the faucet of immigration to turn on and off," he told the New York Times.

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