Yesterday, the CDC released its provisional birth data for 2025. The headline number: 3,606,400 babies born in the United States last year — a 1% decline from 2024, and the fewest births recorded in this country since 1979.
If you follow demographic reporting, the top-line framing was predictable. Economics. Housing costs. Cultural shifts. Women choosing careers over early motherhood. All of which are real. But most of the coverage stopped there — at the sociological layer — and never asked a question I think deserves a lot more attention:
What if the biology is also changing?
I’m a longevity medicine physician. I spend my days thinking about cellular aging, metabolic health, and how environmental exposures quietly erode the body’s systems over decades. And when I look at the fertility data through that lens, I see something the mainstream conversation is largely missing: this isn’t just a story about choice. It’s also a story about physiology. About endocrine disruption, metabolic dysfunction, and a toxic environment that is degrading human reproductive capacity in ways we’re only beginning to understand.
Let me walk you through what the data actually shows, and then I want to talk about what I think is really going on.
The Numbers at a Glance
The general fertility rate in 2025 was 53.1 births per 1,000 women ages 15–44. That’s the lowest on record. Since 2007, the fertility rate has dropped 23% — nearly a quarter — and the pace of decline hasn’t meaningfully slowed.
A few numbers worth sitting with from the NCHS report:
Teen birth rates hit another record low. The rate for 15–19 year-olds dropped 7% in a single year, to 11.7 per 1,000. Since 1991, that’s an 81% decline. This is unambiguously good public health news — better access to contraception, better education, fewer adolescents becoming parents before they’re ready.
Women under 30 are having dramatically fewer babies. Birth rates dropped among every age group under 30. The 20–24 bracket fell from 55.8 to 52.2, and the 25–29 bracket fell from 89.5 to 85.6. Census data show that 63% of women ages 25–29 are now childless, up from about 50% a decade ago.
Women over 30 are having slightly more. The 30–34 bracket ticked up from 93.7 to 96.2, and the 35–39 group rose from 54.3 to 55.1. Births to women 40–44 also increased marginally. But these gains aren’t anywhere close to offsetting the declines in younger cohorts.
C-section rates crept up again. The overall cesarean delivery rate rose to 32.5% — the highest since 2013 — and the low-risk C-section rate hit 26.9%, its highest mark since 2012. The preterm birth rate was unchanged at 10.41%.
The standard interpretation of this data is sociological: people are choosing to have fewer children, later, because life is expensive and priorities have shifted. And that’s true. But I want to push past that and talk about the biological substrates that nobody in the mainstream press seems willing to discuss.
The Sperm Crisis Nobody Wants to Talk About
Let’s start with the male side, because it’s arguably the most alarming piece of this puzzle.
Dr. Shanna Swan, a reproductive epidemiologist at Mount Sinai, published a landmark meta-analysis showing that sperm concentrations in Western men declined roughly 50% between 1973 and 2011. An updated analysis through 2018 showed the trend accelerating — the rate of decline has actually increased to more than 2.6% per year in studies after 2000. If you extrapolate that curve, you land on functionally zero by mid-century.
Now, extrapolations aren’t destiny. But the direction is unmistakable. And the leading suspects aren’t lifestyle alone. They’re chemical.
Endocrine-Disrupting Chemicals: The Invisible Assault
The evidence linking endocrine-disrupting chemicals (EDCs) to male reproductive decline has grown from suggestive to substantial. A 2025 review in Frontiers in Endocrinology documented that environmental EDCs — including phthalates, bisphenol A, PFAS, pesticides, and heavy metals — are associated with roughly a 50% reduction in sperm concentration over the past four decades, a timeline that closely mirrors the exponential increase in EDC production and environmental distribution.
The mechanisms are multiple and they’re well-characterized at this point:
Receptor disruption. EDCs mimic or block estrogen and androgen receptors, scrambling the hormonal signaling that spermatogenesis depends on. Phthalates, for example, are anti-androgenic — they suppress testosterone in a dose-dependent fashion.
Oxidative stress. BPA and related compounds generate reactive oxygen species in testicular tissue, damaging sperm DNA integrity and triggering apoptosis through the mitochondrial pathway. Infertile men consistently show elevated ROS compared to fertile controls.
Epigenetic transmission. This is the part that should alarm anyone who thinks about long-term biology. Animal models have demonstrated that EDC-induced reproductive damage is transgenerational — passed through the male germline via DNA methylation changes. We’re not just talking about one generation’s exposure. We’re talking about inherited impairment.
HPG axis disruption. EDCs can interfere at every level of the hypothalamic-pituitary-gonadal axis, altering GnRH pulsatility, gonadotropin secretion, and downstream steroidogenesis. The net effect is a slow erosion of the hormonal architecture that supports reproduction.
Dr. Swan’s Netflix documentary “The Plastic Detox,” released just last month, followed couples with unexplained infertility who reduced their plastic and chemical exposures for 12 weeks. The results, published in Toxins, showed substantial declines in urinary phthalate and BPA levels, and three of the five participating couples conceived. It’s a pilot study — not proof — but it’s a signal worth paying attention to.
And here’s the thing: microplastics have now been found in human placentas, semen, amniotic fluid, and breast milk. We are, quite literally, marinating in these compounds from before birth to death. If you’re not factoring this into your understanding of declining fertility, you’re missing a big part of the picture.
The Metabolic Engine of Infertility
Now let’s turn to the metabolic side, which connects directly to my practice as a longevity physician.
Roughly 42% of American adults are obese. Another 31% are overweight. That means nearly three-quarters of the reproductive-age population is carrying excess adiposity — and adiposity is not metabolically inert. It’s an active endocrine organ, and it’s profoundly hostile to reproductive function.
In Women: The PCOS–Insulin Resistance Axis
Obesity drives insulin resistance. Insulin resistance drives hyperinsulinemia. And hyperinsulinemia does two devastating things to the female reproductive system: it stimulates ovarian theca cells to overproduce androgens, creating a hyperandrogenic state, and it suppresses hepatic production of sex hormone-binding globulin (SHBG), leaving more free androgens circulating to disrupt ovulation.
The result is a well-worn clinical pathway: anovulatory cycles, menstrual irregularity, and subfertility. PCOS, the most common endocrine cause of female infertility, occurs in 30–75% of obese women. And the relationship is bidirectional — PCOS itself promotes further weight gain, creating a metabolic trap that’s hard to escape.
But it goes beyond PCOS. Obesity-associated chronic inflammation — elevated TNF-α, IL-6, and other pro-inflammatory adipokines — damages oocyte quality, impairs endometrial receptivity, and increases miscarriage risk. Adiponectin, which normally sensitizes tissues to insulin and has anti-inflammatory properties, drops as BMI rises, further destabilizing the hypothalamic-pituitary-ovarian axis. Even “metabolically healthy” obese women have been shown to carry a higher risk of infertility in recent studies.
In Men: Metabolic Obesity and Hypogonadism
The male side is equally stark. Obesity is associated with male obesity-related secondary hypogonadism (MOSH) — a suppression of the HPG axis driven by excess aromatization of testosterone to estradiol in adipose tissue. The result: low testosterone, elevated estrogen, impaired spermatogenesis.
And this intersects with the EDC story in a vicious synergy. Endocrine disruptors are lipophilic — they accumulate in fat tissue. The more adiposity you carry, the larger your reservoir of stored EDCs, and the greater your cumulative exposure. Obesity doesn’t just impair fertility through hormonal disruption — it amplifies the chemical assault.
The Delayed Parenthood Trap
The CDC data shows clearly that women are shifting childbearing into their 30s and 40s. The fertility rate for 30–34 year-olds is now the highest of any age group at 96.2 per 1,000 women, and rates for women 35+ continue to climb. Census data show that half of American women now enter their 30s without having had a child.
From a longevity medicine perspective, I don’t have a problem with delayed parenthood in principle — assuming the biology cooperates. But that’s a big assumption.
Female fertility declines with age. Ovarian reserve diminishes. Oocyte quality degrades, with increasing rates of aneuploidy. These are biological facts, not value judgments. The challenge is that the same metabolic and environmental factors degrading fertility in younger populations don’t spare older ones — they compound. A 35-year-old woman with insulin resistance, years of cumulative EDC exposure, and subclinical inflammation is not operating from the same biological baseline as her grandmother at the same age.
And ART, while a valuable tool, isn’t the safety net many people assume it is. IVF success rates hover around 30%, and they decline sharply after 35. The notion that technology can fully compensate for biological degradation is a cultural myth that does real harm by encouraging complacency.
What a Longevity Medicine Physician Sees
When I look at the 2025 birth data, I don’t just see a demographic trend. I see the downstream expression of several of the hallmarks of aging and metabolic dysfunction acting at the population level:
Chronic inflammation. Both obesity-driven and EDC-driven inflammation impair gonadal function and gamete quality. This is the same inflammaging we talk about in longevity medicine — just showing up earlier, in the reproductive system.
Mitochondrial dysfunction. Oocyte quality is overwhelmingly dependent on mitochondrial health. Oxidative stress from EDCs, metabolic dysfunction, and poor nutrition degrades mitochondrial function in reproductive tissue just as it does everywhere else in the body.
Epigenetic dysregulation. The transgenerational effects of EDC exposure — altered DNA methylation in the germline — represent exactly the kind of epigenetic drift that longevity science is trying to understand and reverse. We may be inheriting compromised reproductive capacity from our parents’ and grandparents’ chemical exposures.
Hormonal signaling breakdown. The HPG axis disruption caused by both metabolic disease and EDC exposure is a hormonal version of the signaling degradation we see across aging tissues. Fertility is, in many ways, a canary in the coal mine for systemic health.
This last point is worth emphasizing. Research has shown that semen quality is a reliable predictor of subsequent morbidity — men with poor sperm parameters have higher rates of cardiovascular disease, metabolic disease, and overall mortality. Fertility isn’t just a reproductive metric. It’s a proxy for whole-body health.
What We Can Actually Do About It
I’m not here to catastrophize. I’m here to be useful. So let me lay out what I think matters from a clinical and practical standpoint.
Reduce EDC Exposure
This is low-hanging fruit, and the pilot data from Swan’s documentary trial supports it. Swap plastic food containers for glass or stainless steel. Stop heating food in plastic. Choose fragrance-free personal care products. Filter your drinking water. Avoid non-stick cookware. These aren’t extreme measures — they’re basic environmental hygiene.
Fix the Metabolic Machinery
If you’re carrying excess weight, especially visceral fat, addressing insulin resistance is the single most impactful thing you can do for fertility — male or female. This doesn’t require heroics. It requires consistent nutritional quality, regular exercise (particularly resistance training), adequate sleep, and in some cases, pharmacological support. Metformin has established evidence for improving ovulation in PCOS, and GLP-1 receptor agonists are emerging as promising tools for obesity-associated subfertility, though the data on live birth rates is still maturing.
Think About Fertility Earlier
I’m not telling anyone when to have children. But I am saying that the biology doesn’t wait for your timeline. If you’re planning to delay parenthood, get a baseline assessment — AMH, FSH, antral follicle count for women; semen analysis for men — earlier rather than later. Knowledge is power, and these are inexpensive tests that can profoundly influence planning.
Advocate for Policy Change
The regulatory environment around EDCs is shockingly inadequate. We’re essentially running an uncontrolled experiment on the human endocrine system, with tens of thousands of chemicals in consumer products that have never been tested for reproductive safety. This should be a bipartisan issue. As Swan puts it: we need to stop using ourselves and our unborn children as lab rats.
The Bottom Line
The 2025 CDC birth data is being framed as a story about economics and lifestyle choice. And those factors are real — housing costs, childcare expenses, shifting cultural norms all play a role. A 2025 BYU survey found that 71% of adults disagreed that having children is affordable for most people. That matters.
But the biological dimension is real too, and it’s being systematically underweighted in the public conversation. Sperm counts are plummeting. Metabolic disease is rampant. Endocrine-disrupting chemicals are ubiquitous. And all of these factors interact in ways that compound over time and across generations.
As a longevity physician, I think about fertility the way I think about healthspan: it’s a systems-level outcome that reflects the integrity of your metabolic, hormonal, inflammatory, and mitochondrial pathways. When those systems degrade — through poor nutrition, chemical exposure, sedentary behavior, or cumulative environmental insult — fertility is one of the first things to go.
The 700,000 fewer babies born last year compared to 2007 aren’t all just “choice.” Some of them are biology. And that’s the part we actually have tools to address.
We just have to start taking it seriously.
Sources & References
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