About 1 in 6 people worldwide experience infertility, defined as the inability to achieve pregnancy after 12 months of regular unprotected sex. The causes split roughly evenly between male and female factors, with a significant portion involving both partners or remaining unexplained. What’s striking is how many different biological systems need to work in sync for conception to happen, and how a disruption in any one of them can prevent it.
Hormonal Signals That Control Fertility
Reproduction depends on a precise feedback loop between the brain and the reproductive organs. A region of the brain releases a signaling hormone that tells the pituitary gland to produce two other hormones, which in turn trigger the ovaries to release eggs or the testes to produce sperm. If any link in that chain misfires, the whole process stalls.
Polycystic ovary syndrome (PCOS) is one of the most common examples. In PCOS, the ovaries produce excess androgens (often thought of as “male” hormones, though all bodies make them). That androgen excess disrupts ovulation, but it also damages fertility in less obvious ways: it reduces the uterine lining’s ability to accept an embryo, triggers chronic inflammation in the uterus, and interferes with how cells there use insulin. Insulin resistance and inflammation then feed each other in a cycle that further degrades the conditions needed for implantation.
Primary ovarian insufficiency is another hormonal condition where the ovaries stop functioning normally before age 40, sometimes in a woman’s twenties. And in men, the same brain-to-gland signaling pathway can fail, leading to extremely low or absent sperm production.
Structural and Physical Causes
Sometimes the reproductive organs themselves are the problem. In women, endometriosis (where tissue similar to the uterine lining grows outside the uterus) can cause scarring that blocks the fallopian tubes or creates an inflammatory environment hostile to eggs and embryos. Fibroids, polyps, or an unusually shaped uterus can also prevent implantation.
In men, the most common reversible physical cause is a varicocele, a swelling of the veins that drain the testicle. Varicoceles appear to raise the temperature around the testes or alter blood flow in ways that reduce both sperm count and sperm quality. Blockages in the ducts that transport sperm are another structural issue. These can result from prior infections, surgeries, or genetic conditions like cystic fibrosis, which can cause the sperm-transporting ducts to be absent entirely.
Retrograde ejaculation, where semen flows backward into the bladder during orgasm rather than exiting the body, is a less common but real physical barrier. Diabetes, spinal injuries, and certain surgeries can cause it.
Age and Egg Quality
A woman’s fertility declines with age in a way that’s steep and not always intuitive. In the twenties and early thirties, the chance of conceiving in any given menstrual cycle is about 1 in 4. By age 40, that drops to roughly 1 in 10. The decline isn’t just about having fewer eggs. The eggs that remain are more likely to carry chromosomal errors, which makes conception harder and miscarriage more common.
Men’s fertility also declines with age, though more gradually. Sperm count, motility (how well sperm swim), and DNA integrity all worsen over time. A healthy sperm count is at least 15 million per milliliter of semen. Men who fall below that threshold face reduced odds of conception regardless of their partner’s age.
Genetic and Chromosomal Factors
Some people are infertile because of conditions written into their DNA. Turner syndrome, where a woman is missing one X chromosome, typically causes the ovaries to develop incompletely. Klinefelter syndrome, where a man has an extra X chromosome, is associated with low or absent sperm production. Deletions on the Y chromosome can also eliminate sperm production entirely.
Single-gene defects play a role too. Mutations in the cystic fibrosis gene, even in men who don’t have full-blown cystic fibrosis, can result in missing sperm ducts. Other inherited conditions like sickle cell disease and thalassemias can impair fertility through their effects on overall health and blood flow to reproductive organs. In women, chromosomal abnormalities are more closely linked to recurrent miscarriage than to difficulty conceiving in the first place, though the practical result is the same: no successful pregnancy.
Infections and Immune Responses
Certain infections directly damage reproductive tissue. Sexually transmitted infections like chlamydia and gonorrhea can scar the fallopian tubes in women or the epididymis (the coiled tube where sperm mature) in men. Inflammation of the testicles, sometimes caused by mumps, can permanently reduce sperm production.
The immune system itself occasionally turns against fertility. Some men produce anti-sperm antibodies, where the immune system mistakenly identifies sperm as foreign invaders and attacks them. This can happen after injury, surgery, or infection and makes it difficult for sperm to survive long enough to reach an egg.
Environmental Toxins and Lifestyle
The modern environment contains chemicals that interfere with reproductive hormones. Phthalates, found in plastics, fragrances, and food packaging, have strong anti-androgenic effects. They can inhibit the growth of developing egg follicles through oxidative stress and disrupt hormonal signaling in both sexes. Bisphenols (like BPA) mimic estrogen, which throws off the delicate balance of hormones that govern whether follicles in the ovary survive or die off prematurely. Both types of chemicals alter gene expression in reproductive cells through multiple pathways.
Industrial chemicals, pesticides, herbicides, organic solvents, and heavy metals like lead have all been linked to reduced sperm counts with extended exposure. Radiation and X-rays can damage reproductive cells directly. Heat exposure matters too: occupations or habits that keep the testes at elevated temperatures (prolonged laptop use on the lap, frequent hot tub use, long hours of driving) can temporarily or permanently lower sperm production.
Lifestyle factors compound the picture. Obesity disrupts hormone levels in both men and women. Smoking damages eggs, reduces sperm quality, and accelerates the age-related fertility decline. Heavy alcohol use suppresses the hormonal signals from the brain that drive sperm and egg production. Even chronic stress can suppress ovulation by disrupting the same brain-to-ovary signaling pathway that governs the menstrual cycle.
Cancer Treatment
Chemotherapy and radiation therapy can severely damage or destroy the cells that produce eggs and sperm. For women, this can trigger early menopause. For men, it can reduce sperm counts to zero. The extent of the damage depends on the type and dose of treatment. Some people recover fertility months or years after treatment ends, while others do not. Surgery for reproductive cancers, including testicular, prostate, and rectal cancers, can also sever or remove the structures needed for natural conception.
Unexplained Infertility
Even after thorough testing, a meaningful share of couples receive no clear diagnosis. Standard fertility workups check hormone levels, ovulation, fallopian tube patency, and semen analysis, but these tests can’t evaluate everything. Subtle problems with egg quality, sperm function at the molecular level, or the uterine lining’s receptivity to an embryo may be at play. Current testing simply can’t detect these issues reliably. For couples in this category, treatment often proceeds empirically, starting with less invasive approaches and escalating based on results rather than a specific diagnosis.
Why Infertility Rates Are Similar Worldwide
One counterintuitive finding from a 2023 WHO analysis: infertility rates are remarkably consistent across income levels. High-income countries show a lifetime prevalence of 17.8%, while low- and middle-income countries come in at 16.5%. The causes may differ (more untreated infections in some regions, more age-related infertility in others), but the overall burden is nearly universal. Infertility is not a problem of wealth or poverty. It’s a fundamental vulnerability of human reproduction itself.