THURSDAY, Nov. 17, 2016 (HealthDay News) -- In what's believed to be the first study of its kind, research suggests that women who give birth for the first time at age 25 or older are more likely to live to 90.

The researchers also found that women who survived to 90 were more likely to be college graduates, married and have a higher income.

"Our study results don't suggest women should delay childbearing, because it's not clearly known why the results suggest [the link to] longevity," said study author Aladdin Shadyab. He's a postdoctoral fellow at the University of California, San Diego School of Medicine.

"Also, a likely explanation is that women who have a child at an older age are usually of higher social and economic status," Shadyab added. "We know from [prior] research that these people are also likely to live longer."

While the average American woman giving birth for the first time today is just over 26 years old -- hardly considered "older" by many societal benchmarks -- the age at first childbirth has continued to rise in the United States.

Rates of first births to women aged 40 and 44, for example, more than doubled between 1990 and 2012, according to the U.S. Centers for Disease Control and Prevention.

Research published in 2015 found that women who gave birth to their last child after age 33 were twice as likely to live to at least age 95 as women doing so by age 29. But scientists had not previously evaluated longevity based on age at first childbirth, Shadyab said.

For the new study, the investigators examined data on about 20,000 women gathered as part of a long-term national study that began in 1993. The women were tracked for up to 21 years, and 54 percent survived to 90 years old.

Women giving birth for the first time at age 25 or older were 21 percent more likely to live to age 90 than those giving birth at earlier ages, the study found. White women with between two and four full-term pregnancies also had higher odds of longevity compared to those with one full-term pregnancy, the findings showed.

"Reproductive factors rarely receive attention in relation to longevity," Shadyab said, noting that he isn't aware of similar data studying fathers. "Usually research is focused on physical activity, diet and [other factors] related to longevity, but [not] reproductive factors," he added.

A U.S. longevity expert said he wasn't surprised by the research findings. Steven Austad is scientific director of the American Federation for Aging Research. He said that, when the study began, the average age of the women was about 75 years -- an age when one-third of their peers would already have died.

"Of the two-thirds left that were studied, more than half lived to 90. If you're not in the longevity business, that sounds like it should be a lot -- but if you live to 75, you have a good chance of living to 90," said Austad. He's also chair of biology at University of Alabama at Birmingham.

The study also reported that "if you're well-educated and well-off and not obese and you don't smoke, you're likely to live a long life," he added. "Which is true, but not very new."

Austad noted that age 25 isn't "particularly late" for giving birth for the first time and agreed with the study authors that women shouldn't plan childbirth timing in hopes of living longer.

The study was published online Nov. 17 in the American Journal of Public Health.

More information

Visit the American College of Obstetricians and Gynecologists to learn more about being an older age at childbirth.