How Fertility is Limited by Age, Visualized

Why it is so crucial to educate young people about the narrow fertility window of youth

Fertility plunged in the United States from about 2.05 births per woman in 2000 to just 1.6 births per woman in 2020, well below replacement amid a global crisis of low birthrates. But fertility desires did not change much at all during that time. Ideal fertility according to surveys has stayed around 2.5 births per woman since 1970, and perhaps has even increased a bit in recent years.

So why did births fall so much when fertility desires stayed about the same? Yale professor Paul Goldsmith-Pinkham recently posted great visuals that plot fertility across different ages over time. They offer a big clue about why women are not having all the children that they want to have.

In this chart, the most yellow line is birth odds by age in 2000. The most violet line is births odds by age in 2020. The area under the curve is proportional to the total fertility rate in that year. What has happened? The left part of the fertility curve has shifted more and more to the right as women avoided teen pregnancy, postponed family, and had children later than before. Meanwhile, the right part of the curve changed very little because women aren’t actually able to increase fertility that much in their 30s and beyond. As a result, later childbearing does not on average make up for children not born to women in their 20s.

What if we look only at women who went to college? College-educated women avoided teen pregnancy even back in 2000 and are more conscientious about education and planning for family. These women started families around three years later in 2020 than in 2000, as the left part of the curves show. But again, the right part of the curve was still almost the same in 2020 as in 2000. Instead of shifting neatly to the right, the curve just got narrower, and the total fertility rate (represented by the area under the curve) kept going down.

In other words, delayed fertility could not be made up. Even though women’s plans have greatly changed, physical fertility limits have hardly changed at all. (American women in 2020 had an ideal fertility that is almost a whole child more than actual fertility, 2.5 vs. 1.6).

Think about what this means! Birthrates, now much too low, would likely be quite a lot higher if women weren’t constrained by age.

Technology has not been able to extend a woman’s reproductive window that much

As the next chart shows, IVF success rates have improved quite a lot for women in their 30s. Meanwhile, women over age 42 continue to have a low success rate from IVF. When older women do get pregnant using IVF, it is often with a donor egg. As much as we want to believe that technology overcomes age-related fertility decline, that hasn't happened very much yet.

The gap between actual fertility and what most of us think about fertility is enormous

With key education, more young people can plan accordingly and have the children they want. There is no substitute for starting earlier.

Consider two final charts. The first chart is from a 2023 working paper by Spears et al. entitled Age and Infertility Revisited that examined the odds of conception according to age. (This paper is part of the Elon Musk funded Population Wellbeing Initiative at the University of Texas at Austin.) The second chart shows results of a survey on fertility knowledge compiled by demographer Lyman Stone.

The first chart is groundbreaking and will hopefully one day become common knowledge. It is new to almost everyone. Most people think fertility drops off somewhere in the late 30s or so. Together with the second chart, this is the story of how almost all of us have a big knowledge gap around human fertility and how crucial the 20s are for having children.

Only 10% of women are aware that fertility decline is noticeable before age 30, while almost half of women think fertility decline becomes noticeable at 40 or later. But by then, the fertility window is almost closed for most women.

These facts are sad and promising at the same time. Sad because almost all of us are close to people who wanted to have children but started too late, if that isn’t your own story. Promising because things could be better for future generations who will be able to plan around peak fertility, which is far earlier than most of us realized.

It will take big changes in timelines around education, partnering and more but we must strive for a world where people can have the children they want to have, and fertility rates rise to where they are healthy and sustaining for society.