Male infertility usually comes down to problems with sperm: too few, too slow, or shaped abnormally. In roughly half of all couples struggling to conceive, a male factor is part of the picture. The causes range from structural issues and hormonal imbalances to genetics, infections, lifestyle habits, and chemical exposures. In 35 to 60 percent of cases, no clear cause is ever identified, though that number shrinks with more thorough testing.
Varicoceles
A varicocele is a swelling of the veins that drain the testicle, similar to a varicose vein in the leg. It’s the single most common identifiable cause of male infertility, found in about 35 percent of men with primary infertility (meaning they’ve never fathered a child) and up to 80 percent of men with secondary infertility (meaning they have before but now can’t). The leading explanation is heat. The scrotum normally stays about 2.2°C cooler than the core body, and sperm production depends on that temperature gap. A varicocele can raise scrotal temperature by 2.6°C, essentially erasing the cooling advantage and disrupting sperm development. Varicoceles are treatable with a minor surgical procedure, and sperm quality often improves afterward.
Hormonal Imbalances
Sperm production runs on a hormone chain. The brain’s hypothalamus releases a signaling hormone that tells the pituitary gland to produce two key hormones: one that stimulates the testicles to make testosterone and another that directly supports sperm production. A disruption anywhere along this chain can reduce or halt fertility.
When the problem originates in the testicles themselves, it’s called primary hypogonadism. Causes include undescended testicles that weren’t corrected in early childhood, injury to both testicles, excess iron in the blood (hemochromatosis), and damage from chemotherapy or radiation. When the problem starts in the brain, it’s called secondary hypogonadism. Pituitary tumors, certain inflammatory diseases like sarcoidosis or tuberculosis, and HIV/AIDS can all interfere with the hormonal signals the testicles need.
Some medications directly suppress testosterone. Opioid painkillers are a well-known culprit, as is exogenous testosterone itself. Men who take testosterone for low energy or muscle building often don’t realize it shuts down the brain’s signals to the testicles, which can drive sperm counts to zero. Sleep apnea, if untreated, is another recognized contributor to low testosterone and impaired fertility.
Genetic Causes
The most common genetic cause of male infertility is Klinefelter syndrome, in which a man carries an extra X chromosome (47,XXY instead of the typical 46,XY). This leads to small testicles, low testosterone, and severely reduced or absent sperm production. It’s also the most frequent chromosomal abnormality found in men with no sperm in their ejaculate.
The Y chromosome contains a region called the azoospermia factor (AZF), which holds genes essential for making sperm. Tiny deletions in this region account for 1 to 18 percent of otherwise unexplained male infertility, depending on the population studied. The AZF region has three subregions (a, b, and c), and the outlook depends on which section is deleted. Deletions in the “a” or “b” regions typically mean no sperm can be retrieved at all. Deletions in the “c” region, which make up about 60 percent of all AZF deletions, are more favorable: sperm can be surgically retrieved in roughly 72 percent of these men, making assisted reproduction possible.
Infections
Infections can damage the reproductive tract in ways that are sometimes permanent. Mumps is the classic example. When mumps strikes after puberty and inflames the testicles (a complication called orchitis), roughly 30 percent of affected men end up with lasting infertility or subfertility. The virus damages the cells that produce sperm, and the resulting scarring can be irreversible.
Sexually transmitted infections like chlamydia and gonorrhea can cause inflammation in the epididymis or the ejaculatory ducts, potentially creating blockages that prevent sperm from reaching the ejaculate. Even after the infection clears, the scarring may remain. Prostate infections can also alter semen quality by changing its chemical composition.
Obesity and Metabolic Health
Excess body fat affects male fertility through several overlapping mechanisms. Fat tissue contains an enzyme that converts testosterone into estrogen, so men with significant obesity tend to have lower testosterone and higher estrogen levels. This hormonal shift alone can suppress sperm production. On top of that, obesity promotes chronic inflammation, insulin resistance, and oxidative stress, all of which can damage sperm DNA directly.
There’s also a thermal component. Increased fat deposits around the thighs and groin raise scrotal temperature. Obesity is linked to a higher incidence of sleep apnea as well, which independently contributes to low testosterone. The relationship between BMI and specific sperm counts is complex (some studies find no clear-cut difference in raw sperm numbers between obese and non-obese men), but the overall effect on fertility is consistently negative when you account for hormone profiles, sperm DNA integrity, and sexual function.
Heat Exposure and Daily Habits
The testicles are outside the body for a reason: they need to stay cool. Anything that consistently raises scrotal temperature can impair sperm production, and the effects tend to worsen with years of exposure.
Professional drivers and men with long daily commutes show poorer sperm quality, and the problem gets more pronounced the longer they’ve been doing it. Workers exposed to intense heat sources, such as bakers, ceramic oven operators, and even submarine engineers stationed near the engine room, take longer to achieve pregnancy with their partners. Sauna use raises scrotal temperature to near core body temperature within about 10 minutes, and regular sessions can cause abnormal sperm counts and reduced motility. These changes are typically reversible once the exposure stops.
Using a laptop on your lap forces your legs together and positions a heat source directly over the groin, both of which raise scrotal temperature. Cycling can have a similar effect depending on duration, intensity, and the type of saddle. Even clothing matters: scrotal temperature is lowest when the testicles hang freely, and tight-fitting underwear or pants reduce the body’s ability to dissipate heat.
Environmental Chemicals
A growing body of evidence links everyday chemical exposures to measurable declines in sperm quality. These substances, broadly called endocrine disruptors, interfere with the hormonal signals that govern reproduction.
Bisphenol A (BPA), found in certain plastics, food can linings, and thermal receipt paper, is associated with lower sperm concentration, reduced motility, and higher rates of abnormally shaped sperm. Its replacement, bisphenol S (BPS), doesn’t appear to be much safer: men with detectable levels of BPS have lower semen volume and sperm counts compared to men without.
Phthalates, used to soften plastics and found in personal care products, flooring, and food packaging, are linked to higher rates of abnormal sperm shape. Pesticide residues tell a similar story. Pyrethroid insecticides, commonly used in household bug sprays and on conventionally grown produce, are associated with increased sperm abnormalities. Organochlorine compounds, older pesticides like DDT that persist in the environment for decades, have been tied to lower sperm motility and abnormal morphology even at background exposure levels. Perfluorinated compounds (found in nonstick cookware and water-resistant fabrics) and flame retardants (PBDEs, found in furniture and electronics) round out the list, both linked to reduced percentages of normally shaped sperm.
Other Medical Causes
Blockages in the reproductive tract can prevent sperm from reaching the ejaculate even when the testicles produce them normally. These obstructions can result from prior infections, surgical complications, or congenital conditions. Men born without the vas deferens, the tube that carries sperm from the testicle, often carry a gene mutation related to cystic fibrosis.
Retrograde ejaculation, where semen flows backward into the bladder instead of out through the penis, is another structural cause. It can result from diabetes, spinal injuries, or certain medications. Cancer treatments deserve special mention: both chemotherapy and radiation can severely damage sperm-producing cells. Some men recover sperm production months or years after treatment, but others do not, which is why sperm banking before treatment is routinely recommended.
When No Cause Is Found
Despite thorough evaluation, somewhere between 35 and 60 percent of infertile men receive a diagnosis of “idiopathic” infertility, meaning no identifiable cause is found through standard testing. One large study of over 1,100 infertile men brought that number down to 19 percent by using a more extensive diagnostic workup, suggesting that many “unexplained” cases simply haven’t been investigated deeply enough. Genetic testing, detailed hormone panels, and assessments of sperm DNA damage are narrowing the gap, but a significant portion of cases still fall into this category.