Fertility Decline is Gradual, Not a Sudden Cliff

Radiolab · December 26, 2025 · Listen to Original Episode →

Original Title:

TL;DR

The concept of a "fertility cliff" at age 35 is a mischaracterization; fertility declines gradually, with significant drops only occurring at later ages, not a sudden precipice.
Fertility data from 18th-century French peasants, used in early studies, is an unreliable basis for modern understanding due to lack of birth control and lifestyle differences.
North American fertility data shows a gentler decline than previously suggested, with a 67% conception chance after one year for 37-39 year olds, contrasting with older, less representative studies.
Male fertility also experiences a downward shift around age 35, though it declines at slightly later ages and less steeply than female fertility, requiring longer study periods for precision.
The perception of a "fertility cliff" may stem from 35 being an age where the biological end of fertility (menopause around 50) begins to come into focus, prompting life planning.
While fertility declines gradually, increased risks for miscarriage and genetic abnormalities like Down syndrome do rise with maternal age, necessitating consideration beyond just conception rates.

Deep Dive

The prevailing narrative of a sharp "fertility cliff" at age 35 for women is a mischaracterization of biological reality, driven by outdated and compromised data. While fertility does decline with age, this decrease is gradual, not sudden, with significant variation among individuals and across populations.

The concept of a fertility cliff, often cited as a dramatic, sudden drop-off in a woman's ability to conceive after 35, is largely a myth. Early data supporting this idea, such as studies on 18th-century French peasants, lacked modern scientific rigor and failed to account for factors like birth control. More recent studies, including those involving women seeking fertility treatment, also presented skewed perspectives. However, comprehensive studies, like one conducted in Denmark, reveal a more nuanced picture: fertility begins a gentle, arcing decline around age 30, with a 78% chance of conception after one year for 20-24 year olds, dropping to 72% for 35-40 year olds. This gradual decline, rather than a precipice, allows for individual variation and suggests that while risks like miscarriage and genetic abnormalities increase with age, a 50/50 chance of conception remains even into the early 40s. Even male fertility shows a similar, though slightly later and less steep, decline after age 35, indicating that the reproductive timeline is a complex interplay of factors, not a single, sharp cutoff. The persistent "cliff" narrative, however, has real-world implications, influencing personal decisions and contributing to the medical classification of "advanced maternal age" at 35, which, while acknowledging increased risks, often overshadows the more gradual biological reality.

Action Items

Audit fertility data sources: Compare North American and Danish fertility studies (Wise et al; Dutch fertility research) to identify methodological differences impacting outcome interpretation.
Analyze male fertility decline: Investigate the 35-year-old shift in male fecundability and its implications for reproductive planning, referencing the 12-year data collection period.
Evaluate "geriatric pregnancy" terminology: Assess the shift from "geriatric mother" to "advanced maternal age" and its impact on public perception and medical guidance.
Track fertility trends: Monitor conception rates for 37-39 year olds and over 40s using updated unpublished data (66% and 51% respectively) to inform personal planning.

Key Quotes

"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."

Molly Webster introduces the concept of the "fertility cliff," a commonly heard phrase that suggests a sudden and dramatic decline in female fertility after age 35. Webster notes that this idea was pervasive among her peers and sparked her curiosity to investigate its accuracy.

"And so there were a lot of like very random conversations and one of the things that came up in those conversations um was a number the number was 35 it took me a while to sort of notice the pattern but i realized that everyone was bringing it up in different ways it was like oh i don't know if there's not that much time left or you know women would be like i'm drying up with my biological clock it's ticking there was this idea that just kept coming up over and over again the idea that at the age of 35 your fertility vanishes if you're a woman it just goes up in smoke"

Webster highlights how the number 35 repeatedly surfaced in her conversations about reproduction, often associated with a sense of urgency or a perceived vanishing of fertility. This recurring theme indicated a widespread belief in a biological clock that sharply impacts women's reproductive capabilities around this age.

"And so this graph looks basically like a gently sloped hill the line starts halfway up it goes up actually a little bit more and then it curves gently down arcing to the bottom woohoo there we go there with that fertility doesn't look like a cliff at all i feel good about that does anybody else"

Webster describes a graph from a Danish study, which she presents as a more accurate representation of fertility over time. This graph shows a gentle decline rather than a sharp drop, leading her to feel reassured that the "fertility cliff" narrative might be an exaggeration.

"so on this one your fertility goes up for a little while right 30 it goes up till your 30 and then when you're 35 you're actually just below the fertility you had when you were 20 which seems like that's that seems great -- so i thought all right i'm reacting very emotionally tell me some numbers -- and so they said for 20 to 24 year olds after one year of trying and having regular sex which is actually like a technical term so take that as you will but i won't judge you the chance of being pregnant is 78 and if you're aged 35 to 40 that percentage is 72 that feels like oh i'll take those odds yeah"

Webster shares specific statistics from the Danish study, noting that fertility increases until age 30 and remains high, with a 72% chance of conception for those aged 35-40 after one year of trying. She finds these numbers encouraging, suggesting that fertility does not drastically diminish at 35 as commonly believed.

"so the danish study i think is a is a happier cheerier study and it's true if you look at the numbers for the north american study -- they are they are lower -- so if you're in the 37 to 39 year old age range and you have sex again regularly for one year you have a 67 chance at conception past 40 it drops to 55"

Webster contrasts the findings of the North American study with the Danish one, reporting lower conception rates for North American women. She states that for women aged 37-39, the chance of conception is 67%, dropping to 55% after age 40, indicating regional differences in fertility trends.

"first it's pretty obvious that there is no cliff as another biologist i spoke with said like nothing in biology is a cliff right it's all just like a gradual change it's like a gradual decline with age and in a way there's something about it being gradual it kind of lets you choose to freak out where you want to freak out"

Webster concludes that there is no literal "fertility cliff," citing a biologist's perspective that biological changes are typically gradual. She suggests that the gradual nature of fertility decline allows individuals to manage their anxieties and choose when to feel concerned.

Resources

External Resources

Books

"Thirty Something" by Radiolab - Mentioned as a 2018 live show that was part of a series on reproduction.

Videos & Documentaries

Radiolab Presents: Thirty Something - Mentioned as a YouTube link for a video related to the episode's topic.

Research & Studies

Graph 1: Can assisted reproduction technology compensate for the natural decline in fertility with age? A model assessment - Referenced for data presented in the episode.
Graph 2: Ovarian aging: mechanisms and clinical consequences (Brookmans, et al.) - Discussed as a source for a graph, with a note that the graph was borrowed from an earlier paper.
Delaying childbearing: effect of age on fecundity and outcome of pregnancy (1991) - Referenced as the original source for a graph used in the episode.
Age and fecundability in a North American preconception cohort study (Wise et al; Dutch fertility research) - Cited for data on fertility and age.

Articles & Papers

Predicting Fertility (Magazine article on Lauren Wise’s research) - Suggested for further reading on fertility research.

People

Molly Webster - Senior correspondent who reported on the fertility cliff.
Lauren Wise - Epidemiologist at Boston University who studies fertility and provided data.
Jad Abumrad - Host of Radiolab.
Rachael Cusick - Producer of the "Thirty Something" live event.
Pat Walters - Editor of the "Thirty Something" live event.
Emily - Mentioned as a friend who discussed fertility.
Chloe - Mentioned as a friend who discussed fertility and her father's advice.
Bianca - Mentioned as a young woman who discussed fertility.
Brokmans - Reproductive endocrinologist whose review paper was discussed in relation to fertility graphs.
Latif Nasser - Host of Radiolab.
Heather Radkey - Contributing reporter who brought the menopause story.
Valerie Chen - Mentioned as part of the staff.
Lulu Miller - Host of Radiolab.
Soren Wheeler - Executive editor of Radiolab.
Sarah Sandback - Executive director of Radiolab.
Jeremy Bloom - Staff member at Radiolab.
W Harry Fortuna - Staff member at Radiolab.
David Gabel - Staff member at Radiolab.
Maria Paz Gutierrez - Staff member at Radiolab.
Sindhu Nayan Sambandam - Staff member at Radiolab.
Matt Keilty - Staff member at Radiolab.
Mona Magavkar - Staff member at Radiolab.
Annie McKeown - Staff member at Radiolab.
Alex Nison - Staff member at Radiolab.
Sarah Kerry - Staff member at Radiolab.
Anisa Vitsa - Staff member at Radiolab.
Arianne Wack - Producer of the episode.
Jessica Young - Staff member at Radiolab.
Rebecca Rand - Mentioned as providing help.
Emily Krieger - Fact-checker for the episode.
Anna Pujol Mazzini - Fact-checker for the episode.
Natalie Middleton - Fact-checker for the episode.
Adina - Caller from Greensboro.

Organizations & Institutions

Radiolab - Podcast and production entity.
WNYC - Radio station and home station for Radiolab.
Boston University - Institution where Lauren Wise is an epidemiologist.
The Lab - Membership program for Radiolab listeners.
Simons Foundation - Provided leadership support for Radiolab's science programming.
John Templeton Foundation - Provided leadership support for Radiolab's science programming.
Alfred P. Sloan Foundation - Provided foundational support for Radiolab.
Yale University - Partnered in a research study on women's pleasure.
Indiana University - Partnered in a research study on women's pleasure.

Websites & Online Resources

radiolab.org/series/radiolab-presents-gonads - Series on reproduction.
radiolab.org/podcast/the-menopause-mystery - Radiolab episode about menopause.
youtu.be/LOJVAaSwags?si=czCBraHf1JEqmAQi - YouTube link for "Radiolab Presents: Thirty Something".
zpr.io/ft6dqdbkJnTd - Link to research paper on assisted reproduction technology.
zpr.io/GrPLebynpvxV - Link to research paper on ovarian aging.
zpr.io/whWg2UAZsb6h - Link to 1991 paper on delaying childbearing.
zpr.io/Rmqry4Kd67hY - Link to research on age and fecundability in North America.
radiolab.org/newsletter - Link to sign up for Radiolab's newsletter.
members.radiolab.org - Link to become a member of The Lab.
radiolab.org/donate - Link to make a donation to Radiolab.
brainhealthmatters.com - Website for information on brain health.
betterhelp.com/radiolab - Website for therapy services.
rippling.com/radiolab - Website for Rippling's HR, IT, and finance platform.
omgyes.com - Website featuring research on women's pleasure and intimacy.

Other Resources

Fertility cliff - A colloquial term describing a perceived sharp decline in fertility after age 35.
Geriatric mother - A term previously used to categorize mothers aged 35 and older.
Advanced maternal age - The current term used for mothers aged 35 and older, linked to increased risks.
Snail sex - A topic mentioned as a potential future episode.
Brain balls - A topic mentioned as a potential future episode.