Rita hadn’t planned on having kids until she was finished with her post-medical school residency. When the subject of children was first broached, she and her husband were still completing their residencies at the University of Utah—for surgery and neurology, respectively. That meant a lot of long hours and sleepless nights. No need to add a baby on top of that, they decided. Plus, Rita (not her real name) would only be 31 when they finished the program. That left plenty of time, conventional science says, to start a family. Rita, I should mention, was my longtime penpal—we met at camp when we were 11, and continued corresponding well into college. She was always the more driven one, the better planner.

Yet when we were both 28, I received an email from Rita with an attached photograph showing two pairs of cowboy boots standing on a wooden fence. Between the boots perched a pair of tiny, knit booties. “There’s a new cowpoke riding into town...” read a banner at the bottom of the photo.

I wrote back, asking Rita what happened. Her response: “I went to see my OB/GYN on January, and after my exam, she asked if there was anything else she could do for me. I jokingly replied, ‘No, unless you can tell me how many years I have left to get pregnant!’ And she said, ‘Well ... there is this one test.”

The answer, the doctor explained, lay in an experimental procedure called an anti-Müllerian hormone test. It’s a blood analysis, just like a cholesterol check, but it measures the amount of anti-Müllerian hormone a woman has circulating in her bloodstream. The idea is that a low result could mean an otherwise healthy woman’s window of fertility is narrower than most, and that she should take action right away, whether it’s starting baby-making, or harvesting her eggs to freeze for the future.

So what started with a joke led to the photo of mini-cowboy boots hovering on my computer screen. And for me, this was just the beginning: I took my own anti-Müllerian hormone test, and I researched and wrote this story. To my co-workers, I claimed curiosity at the test’s effectiveness. But deeper down, I really wanted an answer to what could convince a woman—a doctor; my coolheaded, uber-planner friend—to push her baby-making deadline up so drastically. Surely not some test with only an emerging glimmer of supporting scientific evidence?