By Andrea Bartz

If you've ever wondered how healthy you'll be when you throw yourself a big 4-0 birthday bash, you might want to take a look at this. According to surprising new research, when you have your first child—and whether or not you marry—play a big role in your later health... but not in the way you might think.

In a recent study, researchers examined longitudinal data on 3,348 women who were interviewed annually from 1979 through 2008. All the women had had a baby between age 15 and 35, and at age 40, they rated their health on a scale from poor to excellent.

Scientists split the women into three groups: those who had their first births as teens (ages 15-19), early adulthood (20-24), and older (25-35). The result: Women who gave birth when they were 25 or older had better health at age 40 than those who'd been younger moms. In other words, those who had their first kid in their early 20s were no healthier at age 40 than those who'd been teen moms.

“We’ve had all this focus on the bad effects of teen childbearing and never really asked what happens if these teens waited to early adulthood,” lead author Kristi Williams, associate professor of sociology at Ohio State University, said in a press release. “The assumption has been that ‘of course, it is better to wait.’ But at least when it comes to the later health of the mother, that isn’t necessarily true.”

This begs the question whether or not the later baby/better health link was just correlational: Don't educated, wealthier women tend to wait to have children, and have better health? "Our study controls for an extensive group of socioeconomic background variables including the social class of a woman's family of origin, whether she lived with both parents at age 14, her mother's educational level, whether her family lived in poverty, etc.," said Williams. "Of course, one can never be certain that relevant factors have been controlled, [but] our study goes well beyond many others in accounting for most of the known relevant confounders."

In other words, it really is the early birth itself that knocks down your health later. Alarming when one-third of American babies are in that 20 to 24 age group, per a recent National Vital Statistics Report. Researchers aren't sure why having a baby in your early 20s creates this drag on a mom's health, but they point out that a 21-year-old mom may have less family support than a teenage mom, which could create a domino effect in the mother's stress and educational attainment. (Get pregnant at 16, and your parents could be more likely to give you a hand with the whole childrearing thing.)

Things get even wrinklier when we take marriage into account. Conventional wisdom dictates that married women are healthier, right? Williams' team found that, yes, women who were married when they had their first kid reported better health at age 40. But marriage wasn't necessarily the wellness boon you'd expect for women who were unwed when they gave birth.

For black women, those who never married after having a kid outside of wedlock had better health at 40 than those who went on to marry after they'd had a rugrat. Very interesting when you consider costly give-your-kids-a-dad pro-marriage programs.

“Most studies indicate that marriage promotion efforts have been unsuccessful in increasing marriage rates. Our findings suggest that may be a good thing, at least for black women’s health,” Williams said.

Why doesn't marriage improve black mothers' health? While the study didn't directly answer that, Williams points to other research suggesting many black single moms face a lack of eligible men with good economic prospects. Those who do marry are more likely to have a husband who lacks a high school education or a good job—hello, health-hurting stress and marital conflict.