Infertility has dozens of possible causes, ranging from hormonal conditions and infections to everyday habits and chemical exposures. Roughly one in six couples worldwide experiences difficulty conceiving, and the cause is split fairly evenly between male factors, female factors, and a combination of both. Here’s what can actually disrupt your ability to have children, and why.
Hormonal and Ovulation Disorders
The most common cause of female infertility is a problem with ovulation. Polycystic ovary syndrome (PCOS) is the leading culprit. In PCOS, chronic low-grade inflammation in the ovarian tissue combines with insulin resistance and elevated androgen levels to disrupt the normal growth of follicles and the maturation of eggs. The lining of the uterus also becomes less receptive to a fertilized egg, making both releasing an egg and implanting one harder. Other ovulation disorders include thyroid problems (both overactive and underactive) and a condition called premature ovarian insufficiency, where the ovaries stop functioning normally before age 40.
Structural Problems in the Reproductive Tract
Blocked or damaged fallopian tubes prevent sperm from reaching an egg or stop a fertilized egg from traveling to the uterus. Endometriosis, where tissue similar to the uterine lining grows outside the uterus, is one of the most common causes of this kind of structural damage. It can also create scar tissue (adhesions) that distorts the anatomy of the pelvis. Uterine fibroids, polyps, and scarring inside the uterus from previous surgeries can similarly interfere with implantation or block the path an embryo needs to travel.
Pelvic Inflammatory Disease and STIs
Sexually transmitted infections, particularly chlamydia and gonorrhea, can silently damage the reproductive tract if untreated. When bacteria travel upward from the cervix, they cause pelvic inflammatory disease (PID), which scars and blocks the fallopian tubes. The damage is cumulative: after a single episode of PID, up to 12% of women will develop blocked tubes. After two episodes, more than a third are affected. With three episodes, up to 75% of women will have tubal blockages. Many people with chlamydia never have symptoms, which is why the infection can do so much damage before anyone realizes it’s there.
Male Factor Infertility
About half of all infertility cases involve a male factor. The most common physical cause is a varicocele, an enlargement of the veins inside the scrotum. Varicoceles raise the temperature around the testicles and trigger oxidative damage that harms sperm in several ways at once: it reduces sperm count, impairs motility by damaging the energy-producing structures inside sperm cells, and can fragment sperm DNA. Varicoceles are found in roughly 15% of all men but in up to 40% of men being evaluated for infertility.
Other male causes include hormonal imbalances, undescended testicles, blockages in the ducts that carry sperm, and genetic conditions that affect sperm production. Some men produce no sperm at all, a condition called azoospermia, which can result from long-term heavy alcohol use, certain medications, or physical obstruction.
Smoking and Alcohol
Smoking is one of the strongest modifiable risk factors for infertility in both sexes. In men, smokers are roughly 12 times more likely to have abnormally low sperm counts compared to nonsmokers. Cigarette chemicals interfere with reproductive hormone production and cause structural damage to sperm. In women, smoking accelerates the loss of eggs, damages the fallopian tubes, and increases the risk of miscarriage.
Heavy, long-term alcohol use also takes a toll. In men, it can suppress testosterone, shrink the testicles, and in severe cases lead to a complete absence of sperm production. In women, heavy drinking disrupts the menstrual cycle and ovulation. Even moderate drinking may reduce fertility, though the evidence is less clear-cut at lower levels of consumption.
Body Weight
Both extremes of body weight affect fertility. In women, excess body fat increases estrogen production and can disrupt the hormonal signals that trigger ovulation. Being significantly underweight (a BMI below 18.5) can shut down ovulation entirely, because the body interprets low energy reserves as a signal that it’s not a safe time to support a pregnancy. A BMI of 30 or above is classified as obese and is associated with longer time to conception, poorer outcomes with fertility treatments, and higher miscarriage rates.
In men, obesity is linked to lower testosterone, reduced sperm quality, and a faster decline in sperm parameters over time. The effect compounds with age, meaning the combination of being overweight and getting older is particularly damaging to sperm health.
Medications That Affect Fertility
Several common medication classes can reduce fertility, sometimes temporarily and sometimes permanently. Chemotherapy drugs are the most well-known offenders. They work by killing rapidly dividing cells, which includes eggs and sperm-producing cells. Some people recover fertility after treatment, while others do not.
Beyond chemotherapy, other medications that can lower sperm counts or impair sperm movement include drugs used for hair loss and enlarged prostate (like finasteride), certain antidepressants (SSRIs), anti-seizure medications, and some antiretroviral drugs used to treat HIV. Anabolic steroids, often used for bodybuilding, are a major but underappreciated cause of male infertility. They shut down the body’s own testosterone production, which can reduce sperm counts to zero. Recovery after stopping steroids can take months or, in some cases, may be incomplete.
Environmental Chemicals
A growing body of evidence links everyday chemical exposures to fertility problems. Endocrine disruptors are chemicals that interfere with your hormonal system. They’re found in plastics, cosmetics, food packaging, pesticides, and household cleaning products. The most studied include bisphenol A (BPA), phthalates, parabens, and dioxins.
These chemicals can mimic estrogen or block other hormones by binding to the same receptors your natural hormones use. In women, this can disrupt the menstrual cycle, interfere with egg development, and reduce egg quality. Phthalate exposure has been linked to lower fertilization rates. In men, endocrine disruptors are associated with lower sperm counts and altered hormone levels. The challenge is that exposure is nearly constant and comes from many sources simultaneously, making it difficult to eliminate entirely. Reducing use of plastic food containers, choosing fragrance-free personal care products, and eating fewer processed foods are practical ways to limit exposure.
Age
Age is the single most significant factor in female fertility. Women are born with all the eggs they will ever have, and both the number and quality of those eggs decline steadily over time. The decline accelerates sharply after age 35 and becomes steep after 40. This isn’t just about quantity: older eggs are more likely to have chromosomal abnormalities, which increases the risk of failed implantation and miscarriage.
Male fertility also declines with age, though more gradually. Sperm quality, including motility and DNA integrity, decreases over the decades. Men over 40 take longer to conceive with their partners and have higher rates of miscarriage compared to younger men, even when the female partner’s age is accounted for.
Unexplained Infertility
In roughly 10 to 15% of couples, standard testing finds no clear cause. This doesn’t mean nothing is wrong. It often means the problem is subtle enough that current diagnostic tools can’t pinpoint it. Possible hidden factors include minor egg quality issues, sperm DNA damage that doesn’t show up on a standard semen analysis, or problems with how the embryo implants. Endocrine disruptors and oxidative stress may also play a role in cases that appear unexplained.
General guidelines suggest seeing a fertility specialist if you’re under 35 and haven’t conceived after 12 months of unprotected sex, or after 6 months if you’re 35 or older. If you’re 40 or older, starting with a specialist from the outset is reasonable given the steeper decline in fertility at that age.