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New research from �鶹��Ѱ�����Boulder Associate Professor Amanda Stevenson uses math and demography to show women’s ability to get pregnant doesn’t necessarily decrease with age

Amanda Stevenson’s affection for her mentor, fellow demographer Jane Menken, is obvious. She’s quick to rave about Menken’s legacy, as well as her smarts: “She’s a super genius.”

Stevenson, associate professor of sociology at the �鶹��Ѱ�����, speaks especially reverently about a significant paper Menken published in 1985 while serving as president of the Population Association of America ( is now a distinguished professor emerita in sociology and director of the ).��

In the now-40-year-old paper, “”��(published in Science), Menken took a mathematical approach to synthesizing historical and contemporary data about fecundability; her models made it clear that we do not have enough evidence to link a woman’s age to her ability to get pregnant.

Amanda Stevenson is a �鶹��Ѱ�����Boulder associate professor of sociology.��

It boils down to the selection inherently involved in tracking aging women’s fecundability, Menken argued.��

“Fecundability is a measure of your monthly probability of conception when you're trying,” Stevenson says. “But if you get pregnant really easily, then you may never actually have to try to get pregnant. So we would never observe your fecundability in any study, no matter how well designed. Demographers know this means data from conception-seekers cannot tell us when fecundability declines or by how much.”��

This year, Stevenson, along with Menken and��Leslie Root (who serves as associate director of the �鶹��Ѱ�����Population Center), set out to emphasize this fact through a clear mathematical simulation. The model they created illustrates that evidence for age-related fertility decline is weaker than most people—and medical societies—think. The results were published in�� in the Vienna Yearbook of Population Research 2026.��

Righting a wrong

One of Stevenson’s key reasons for undertaking this study is that she constantly sees Menken’s paper cited incorrectly by major medical organizations. “They say that fertility falls off a cliff at 35 because they misinterpret one of the paper’s figures that includes birth data from the 1700s and 1800s,” Stevenson says. “This is, like, misinformation at the highest level.”��

First, it’s important to distinguish between fertility and fecundability. “The thing we call fertility is a colloquial concept that cannot be well defined mathematically,” says Stevenson. “It’s basically all the factors that go into whether or not you can have a baby.”��

Fecundability, on the other hand, is the statistical probability of a woman getting pregnant every cycle. It’s quantifiable, which makes it much easier for demographers like Stevenson to study.��

Stevenson thus set out to create a mathematical simulation to demonstrate that we know less about fecundability than we think we do.��

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“It's not that I would say that fecundability doesn't decline with age,” she says. “The data can show us how much it declines due to selection and age together. But we don't know how much of that is from selection and how much of it is from age.”

She thinks this uncertainty is worth clarifying to women who could be structuring their whole lives around the suspected drop in fecundability around their mid-30s. “Women all over the world, particularly now in this country, are taught to be terrified of these particular ages,” she says.��

A complex method to demonstrate a simple truth��

The simulation Stevenson created to help clarify this concept is a complex mathematical model that illustrates how we could�� see fecundability “decline” with age in a world where no one’s fecundability ever actually declined.��

The results show that, if every woman’s ability to become pregnant stays the same as she ages, conception will still appear to get harder with age simply because fecundability itself influences when someone tries to get pregnant, on average. In a population with more varied levels of fecundability, the age-related dropoff is more apparent.��

“We see it changing with age, but that fact just tells us that people who are trying at different ages have different levels of fecundability,” says Stevenson. “Not that individuals’ fecundability declines as they age.”

It’s worth noting that there are still certain medical issues that can arise in pregnancy for women as they age. Women of “advanced maternal age”—over 35—still carry more risks for things like preeclampsia, gestational diabetes or low birthweight.��

Taking selection out of the equation

Stevenson is actively seeking funding for the next stage of this project, which she believes will provide a clearer picture of age-related fecundability. The study subjects? Women who have not had substantial exposure to sperm prior to trying to become pregnant—in other words, non-heterosexual women. The project would be led by Stevenson and the rest of the team behind the current paper, along with a sociologist who studies queer families.��

“If we had a population who were trying to get pregnant at different ages, but those ages weren’t influenced by fecundability itself, we could potentially uncover a real age pattern,” Stevenson says.��

All of this work is aimed at dialing back the fear that aging women can experience when trying to conceive. “We need to figure out what the real pattern is, because obviously it matters,” Stevenson says. “People need to be able to make informed decisions about their lives.”

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