Male infertility contributes to roughly half of all couples who struggle to conceive, and the causes range from physical problems with the reproductive tract to genetic conditions, hormonal shortfalls, and everyday exposures. In many cases, more than one factor is at play. Understanding what can go wrong helps explain why fertility isn’t guaranteed and what can be done about it.
Varicoceles: The Most Common Physical Cause
A varicocele is a swelling of the veins inside the scrotum, similar to a varicose vein in the leg. It develops when a valve inside the spermatic cord (the tissue that holds each testicle in place) fails to close properly. Blood that should flow back toward the heart pools in the vein instead, and over time the vein swells. Varicoceles contribute to about 40% of all male infertility cases.
The exact way varicoceles damage fertility isn’t fully understood, but the leading theory centers on heat. Pooled blood raises the temperature inside the scrotum, and sperm production is highly sensitive to temperature. Even a small, sustained increase can lower sperm count or impair how well sperm develop. Varicoceles are often discovered during a fertility workup because many men never notice symptoms.
When the Body Doesn’t Produce Enough Sperm
Some men produce little or no sperm at all, a condition called azoospermia when sperm are completely absent from the ejaculate. This splits into two broad categories. In obstructive azoospermia, the testicles make sperm normally, but a physical blockage somewhere along the reproductive tract prevents sperm from reaching the semen. In non-obstructive azoospermia, the testicles themselves fail to produce sperm adequately.
Blockages can occur in the epididymis (the coiled tube where sperm mature), the vas deferens (the duct that carries sperm out), or near the prostate. Causes include prior surgery (a vasectomy is the intentional version), scarring from infections, trauma, or a congenital condition where the vas deferens never formed at all. When the blockage sits higher up in the tract, near the prostate or seminal vesicles, semen volume tends to be noticeably low. When it’s further downstream, semen volume looks normal but still contains no sperm.
Non-obstructive azoospermia is trickier. The testicles are typically smaller than average, and blood tests show elevated levels of follicle-stimulating hormone (FSH), because the pituitary gland keeps ramping up its signal to the testicles without getting a response. The causes include genetic abnormalities, chemotherapy or radiation exposure, infections, and in many cases no identifiable cause at all.
Genetic Conditions That Impair Fertility
Genetics account for a meaningful share of severe male infertility. The most common chromosomal abnormality is Klinefelter syndrome, in which a man carries an extra X chromosome (47,XXY instead of the typical 46,XY). It affects 7 to 13% of men who produce no sperm. Men with Klinefelter syndrome often have smaller testicles, lower testosterone, and may not be diagnosed until they try to conceive.
Another important genetic cause involves tiny deletions on the Y chromosome, in a region called the azoospermia factor (AZF). This region contains genes essential for sperm production. Among men with no sperm or severely low counts, about 16% carry deletions in these AZF regions. When looking only at men with complete absence of sperm, that number rises to 23%. These deletions can be passed to sons conceived through assisted reproduction, so genetic testing before treatment matters.
Beyond these well-known conditions, roughly 1 in 20 men with low sperm counts have other chromosomal rearrangements, such as translocations where segments of chromosomes swap places. These rearrangements can disrupt the precise process of cell division that sperm production requires.
Hormonal Imbalances
Sperm production depends on a chain of hormonal signals that starts in the brain. The hypothalamus signals the pituitary gland, which releases two key hormones: luteinizing hormone (LH) and follicle-stimulating hormone (FSH). LH tells the testicles to produce testosterone, which is critical for sperm development. FSH directly stimulates the cells inside the testicles that support sperm maturation. If either hormone is missing or low, sperm production stalls.
Without LH, testosterone drops too low to sustain the environment sperm need to develop. Without FSH, even adequate testosterone isn’t enough to fully drive sperm production. This type of hormonal failure, called hypogonadotropic hypogonadism, can result from pituitary tumors, head injuries, certain genetic conditions, or the use of external testosterone (more on that below). The encouraging part is that hormonal causes are often treatable, because the testicles themselves are still capable of making sperm once the right signals are restored.
Medications That Suppress Sperm Production
Several common medications can significantly reduce fertility, sometimes without the man realizing it. Testosterone replacement therapy is one of the most important culprits. It’s widely prescribed for low testosterone, but it actually shuts down the brain’s signals to the testicles, causing sperm production to plummet. Infertility is a primary side effect of testosterone treatment, and many men aren’t warned about it before starting.
Anabolic steroids work the same way. They flood the body with synthetic hormones, which tells the pituitary gland to stop sending its own signals to the testicles. The result is often dramatically reduced sperm counts that can take months to recover after stopping.
Antidepressants, particularly SSRIs and tricyclic antidepressants, pose a different set of problems. Medications like fluoxetine, sertraline, and paroxetine can cause sexual dysfunction (lower desire, delayed orgasm, or inability to ejaculate) and have also been linked to lower sperm counts. For men actively trying to conceive, discussing alternatives with a prescriber is worth considering.
Environmental and Lifestyle Factors
The environment a man lives and works in can chip away at sperm quality over time. Research shows strong evidence that exposure to phthalates (found in plastics, personal care products, and food packaging) and pesticides negatively affects male reproductive health. Evidence for other industrial chemicals, including certain flame retardants and nonstick compounds, is less conclusive in humans but consistently harmful in animal studies.
Heat exposure is a straightforward risk. The testicles sit outside the body for a reason: sperm production requires a temperature a few degrees below core body temperature. Prolonged heat from laptops on the lap, frequent hot tub use, tight clothing, or occupational exposure (welding, long-haul driving) can reduce sperm counts. Tobacco use, heavy alcohol consumption, and marijuana have all been linked to decreased fertility as well.
How Male Infertility Is Diagnosed
A fertility evaluation typically starts with a medical history and physical exam. Your doctor will ask about past surgeries, chronic illnesses, medications, sexual habits, and puberty development. A physical exam checks the size and consistency of the testicles and feels for varicoceles.
The cornerstone test is a semen analysis. You provide a sample, and a lab measures sperm count, the percentage of sperm moving well (motility), and how many have a normal shape (morphology). The lab also checks for signs of infection. If the initial results are abnormal, the test is usually repeated to confirm.
From there, testing branches out based on what the semen analysis reveals. Hormone blood tests measure testosterone, FSH, and LH to assess whether the signaling chain from brain to testicles is intact. A scrotal ultrasound can detect varicoceles or structural problems. If sperm count is extremely low or zero, genetic testing looks for Y-chromosome deletions or chromosomal abnormalities like Klinefelter syndrome. In some cases, a post-ejaculation urine test checks whether sperm are traveling backward into the bladder instead of exiting normally, a condition called retrograde ejaculation. A transrectal ultrasound can identify blockages near the prostate or seminal vesicles.
For many men, the evaluation pinpoints a specific, treatable cause. For others, especially those with non-obstructive azoospermia, the path forward involves assisted reproduction techniques or, in some cases, surgical sperm retrieval directly from the testicle. The cause matters because it determines which options are realistic and how likely they are to succeed.