Birth rates have steadily declined in the U.S. for more than a decade, prompting much discussion about the decision to delay motherhood or forgo it altogether—as well as about the complications women may face if they wish to conceive later in life.

For myself and many of my friends, pregnancy was something to avoid throughout our 20s and into our 30s. We had careers to focus on and student loans to pay off. We were swiping on the apps and dating different people. I can't tell you how many times I said to a prospective partner some version of this line: "Sure, I want to have kids someday—but not anytime soon."

I never gave much thought as to when that time might run out.

Women might actually have more time than they used to, according to a recent study published in the Journal of the American Medical Association. The mean reproductive life span—the number of years between menarche (first period) and menopause (the