Male infertility has no single cause. It stems from a wide range of factors, from physical abnormalities and genetic conditions to hormone imbalances, lifestyle habits, and environmental exposures. In roughly half of all couples struggling to conceive, a male factor is involved. Here’s what can go wrong and why.
Varicocele: The Most Common Physical Cause
A varicocele is a swelling of the veins that drain the testicle, similar to a varicose vein in the leg. It’s found in about 10 to 15 percent of men overall, but that number jumps to over 40 percent in men with primary infertility. The problem gets worse with age.
The mechanism is straightforward: those enlarged veins create pockets of pooled blood that raise the temperature inside the scrotum. Testicles need to stay cooler than the rest of the body to produce sperm effectively. The extra heat triggers oxidative stress, which is a buildup of damaging molecules that overwhelms the body’s natural defenses. This damages sperm DNA, reduces how well sperm can swim, and lowers overall semen quality. Varicoceles are treatable with a minor surgical procedure, and many men see improvements in sperm parameters afterward.
How Heat Damages Sperm Production
Testicles sit outside the body for a reason. They normally stay at about 35°C (95°F), roughly two degrees below core body temperature. Even a small increase of a fraction of a degree can impair sperm production. One study comparing nearly 200 men found that infertile men had scrotal temperatures about half a degree higher on average than fertile men.
The sources of that extra heat are often surprisingly mundane. Sitting with your thighs pressed together for an hour raises scrotal temperature by about 2°C. Place a laptop on top of that, and you add another half a degree. Regular hot tub or hot bath use (30 minutes or more per week) has been linked to reduced sperm counts. One study found that sauna use totaling about two and a half hours every two weeks cut sperm counts by up to 50 percent.
The good news is that heat-related damage is usually reversible. After stopping the heat exposure, sperm counts typically recover within about four weeks, though the decline itself can take five to seven weeks to show up. That delay exists because sperm take roughly two to three months to fully develop, so damage from heat today shows up in a semen analysis weeks later.
Genetic Conditions
Some men are born with genetic differences that make producing sperm difficult or impossible. The most common is Klinefelter syndrome, a condition where a man has an extra X chromosome (47,XXY instead of the typical 46,XY). It occurs in about 1 in 600 males and almost always leads to a complete absence of sperm in the ejaculate. Many men with Klinefelter syndrome go through life without being diagnosed, because the physical signs can be subtle.
The second most common genetic cause is Y chromosome microdeletion, where small segments of the Y chromosome are missing. These deletions affect about 7 percent of infertile men globally and directly impair the genes responsible for sperm production. Depending on which segment is deleted, the impact ranges from very low sperm counts to none at all. Genetic testing can identify both conditions and help guide decisions about fertility treatment options like sperm retrieval procedures.
Hormonal Imbalances
Sperm production depends on a chain of hormonal signals. The brain releases signaling hormones (FSH and LH) that tell the testicles to produce testosterone and sperm. When the brain doesn’t send enough of those signals, testosterone drops and sperm production slows or stops entirely. This is called secondary hypogonadism, and it can result from pituitary tumors, head injuries, certain medications, or excess body weight.
The tricky part is that low testosterone alone doesn’t always explain infertility. Some men have normal testosterone levels but still produce inadequate sperm because of problems further down the chain. A full hormonal workup, not just a testosterone test, is needed to identify where the breakdown is occurring.
How Excess Weight Disrupts Hormones
Obesity creates a hormonal environment that actively works against fertility. Fat tissue contains an enzyme called aromatase that converts testosterone into estrogen. The more body fat a man carries, the more aromatase activity he has, which means more testosterone gets converted and less remains available. The resulting rise in estrogen further suppresses sperm production in the testicles.
This creates a feedback loop: higher estrogen promotes more fat storage, which increases aromatase activity, which converts even more testosterone to estrogen. The effect on the testicles is directly inhibitory, reducing both testosterone output and sperm counts. Weight loss can partially reverse this cycle, and some men see meaningful improvements in semen quality after losing a significant amount of weight.
Medications That Impair Fertility
Several common medications can reduce sperm production, sometimes without men realizing it. The most well-known culprit is supplemental testosterone itself. When a man takes testosterone from an outside source (injections, gels, or patches), his brain detects the high levels and shuts down its own signaling to the testicles. Without those signals, the testicles stop making sperm. This is one of the most frequent causes of drug-induced infertility, and it’s entirely reversible once the testosterone is stopped, though recovery can take months.
Anabolic steroids work through the same mechanism, often with more dramatic effects because the doses are higher. Other drug classes linked to impaired fertility include certain antidepressants (SSRIs), opioid pain medications, and medications used for hair loss or prostate conditions. Chemotherapy is a well-known cause that can sometimes permanently damage sperm-producing cells, which is why sperm banking before cancer treatment is standard practice. If you’re trying to conceive and taking any prescription medication, it’s worth checking whether it could be affecting your fertility.
Environmental Chemical Exposure
A growing body of evidence points to environmental chemicals as contributors to declining sperm quality. The strongest human evidence exists for two categories: phthalates, which are found in plastics, personal care products, and food packaging, and pesticides used in agriculture. Both have been associated with reduced sperm concentration and quality in human studies.
Other chemicals under investigation include bisphenols (like BPA, found in can linings and some plastics) and PFAS, the so-called “forever chemicals” found in nonstick cookware, water-resistant clothing, and food wrappers. Animal studies show clear harm from these substances, but the human evidence is still limited. The challenge is that exposure to these chemicals is nearly universal in industrialized countries, making it difficult to study their effects in isolation. Reducing exposure where practical, such as choosing glass or stainless steel food containers and washing produce, is a reasonable precaution.
Other Common Causes
Several other factors round out the picture. Infections like sexually transmitted diseases or mumps after puberty can damage the reproductive tract or the testicles directly. Undescended testicles, a condition present from birth where one or both testicles fail to move into the scrotum, can permanently impair sperm production if not corrected early in childhood. Ejaculatory problems, including retrograde ejaculation (where semen flows backward into the bladder), can prevent sperm from reaching a partner.
Physical blockages in the tubes that carry sperm are another category. These can result from prior surgery, infections, or congenital absence of the vas deferens, a condition linked to the gene that causes cystic fibrosis. In these cases, the testicles produce sperm normally, but the sperm can’t get out. Surgical repair or sperm retrieval procedures can often work around these blockages.
In about 30 to 40 percent of infertile men, no clear cause is ever identified, a frustrating diagnosis called idiopathic infertility. Even in these cases, lifestyle modifications like maintaining a healthy weight, avoiding excess heat, limiting alcohol, and not smoking can improve sperm parameters over the course of several months.