Denmark Study: No Cliff, Gentle Decline

As she -- and her friends — approached the age of 35, senior correspondent Molly Webster kept hearing a phrase over and over: “fertility cliff.” It was a short-hand term to describe what she was told would happen to her fertility after she turned 35 — that is, it would drop off. Suddenly, sharply, dramatically. And this was well before she was supposed to hit menopause. Intrigued, Molly decided to look into it — what was the truth behind this so-called cliff, and when, if so, would she topple? 

This story first premiered in “Thirty Something,” a 2018 Radiolab live show that was part of, Gonads, (https://radiolab.org/series/radiolab-presents-gonads)a six-episode audio and live event series all about reproduction and the parts of us that make more of us. The live event was produced by Rachael Cusick and edited by Pat Walters.

Special thanks to epidemiologist Lauren Wise, at Boston University. Plus, Emily, Chloe, and Bianca. And of course, Jad Abumrad.

If you’re more of a visual person, here are the graphs we explain in the episode, we also include links to the corresponding papers in our Episode Citations Section, below!

LINK TO GRAPHS:
https://media.wnyc.org/i/1860/1046/c/80/2025/12/FERTILITY_AGE_GRAPHS_1-4.jpg

EPISODE CREDITS: 
Reported by - Molly Webster
Produced by - Arianne Wack
Fact-checking by - Diane A. Kelly

EPISODE CITATIONS:

Audio:

• Gonads (https://radiolab.org/series/radiolab-presents-gonads/)
  A six-part audio series on reproduction and the parts of us that make more of us
• The Menopause Mystery (https://radiolab.org/podcast/the-menopause-mystery)
  One of Radiolab’s most listened-to episodes of 2025!
   

Videos:

“Radiolab Presents: Thirty Something”
https://youtu.be/LOJVAaSwags?si=czCBraHf1JEqmAQi

Research Articles:

• Graph 1: Can assisted reproduction technology compensate for the natural decline in fertility with age? A model assessment (https://zpr.io/ft6dqdbkJnTd)
   
• Graph 2: Ovarian aging: mechanisms and clinical consequences (https://zpr.io/GrPLebynpvxV) , Brookmans, et al.
  • BUT, the graph was borrowed and actually comes from this 1991 paper, Delaying childbearing: effect of age on fecundity and outcome of pregnancy” (https://zpr.io/whWg2UAZsb6h) 
     
• Graph 3 and 4: Age and fecundability in a North American preconception cohort study, (https://zpr.io/Rmqry4Kd67hY) Wise et al; Dutch fertility research

Further reading: 
Predicting Fertility, (https://zpr.io/YEdfiYT29rUh): Magazine article on Lauren Wise’s research,
 

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