Most guys don't think about fertility until they're trying to start a family. By then, years of unnoticed warning signs have already passed. The reality is that male fertility has been declining for decades. Sperm counts dropped by more than 50% between 1973 and 2018 according to a meta-analysis published in Human Reproduction Update (Levine et al., 2023). The decline is real, and it starts earlier than most people realize.
The good news? Many fertility issues are detectable long before they become problems. Here is what to watch for and what the research shows.
Physical Signs That Deserve Attention
Undescended Testicles or Late Puberty
If one or both testicles haven't dropped into the scrotum by age one, that is called cryptorchidism. It is the most common birth defect in male reproductive organs, affecting about 3% of full-term newborns. When left untreated, it directly impacts sperm production later in life because your testicles sit outside your core body temperature for a biological purpose-they keep sperm at 94-97°F.
A study in The Journal of Urology found that men with a history of undescended testicles had a 20-30% lower sperm count on average, even after corrective surgery. If a teenage boy never had this corrected, it is worth a conversation with a doctor.
Late puberty is another flag. If a young man hasn't started showing signs of puberty by age 14-voice change, growth of pubic hair, testicular enlargement-it may indicate hormonal issues that could affect fertility. This is not about being a "late bloomer." It is about whether the endocrine system kicked on properly.
Varicocele
This is the most common reversible cause of male infertility, and it often shows up during adolescence. A varicocele is essentially varicose veins in the scrotum. You can feel it-a "bag of worms" sensation above the testicle, usually on the left side.
About 15% of all men have varicoceles, but the number jumps to 40% among men evaluated for infertility. Why? The pooled blood raises scrotal temperature by about 2-3°F, which is enough to impair sperm production. In teenagers, varicoceles can also stunt growth of the affected testicle.
The warning sign is a dull ache or heaviness in the scrotum, especially after standing or physical activity. Sometimes there is no pain at all-just the visible or palpable vein structure. If a young man notices this, it needs a urology exam.
Small or Shrinking Testicles
Testicular size matters for fertility. The average adult testicle is about 4-5 cm long and 2.5-3 cm wide. Anything significantly smaller-or a noticeable decrease in size-can indicate low sperm production or hormonal problems.
The testicles produce both sperm and testosterone. When they shrink, something is off. Common causes include varicocele, hormonal imbalances, past infections like mumps orchitis, or anabolic steroid use (which is more common among young men than most parents want to admit). A 2021 study in Andrology found that testicular volume below 12 mL (measured by ultrasound) was strongly correlated with low sperm counts.
Pain or Swelling
Acute pain in the testicles is a medical emergency. Testicular torsion-where the spermatic cord twists and cuts off blood flow-can destroy a testicle within six hours. If a young man feels sudden, severe pain, he needs to go to the emergency room immediately.
But chronic, low-grade pain or intermittent swelling also matters. It could indicate an infection (epididymitis), a hernia, or a varicocele. Infections of the reproductive tract can cause scarring that blocks sperm from leaving the testicle. Some of these infections are sexually transmitted-chlamydia and gonorrhea can cause epididymitis in young men-and they are treatable, but only if you catch them.
What the Body Is Telling You
Changes in Ejaculate
Most young men have some idea what is normal for them. Changes worth noting include:
- Low volume. The average ejaculate is 1.5 to 5 mL-about a teaspoon. Consistently less than that could mean a blockage or low seminal fluid production.
- No ejaculate at all. This is called azoospermia and requires immediate medical evaluation.
- Blood in the semen. This is usually not serious-often caused by a burst blood vessel-but it should be checked by a doctor to rule out infection or structural problems.
Delayed or Absent Secondary Sexual Characteristics
By age 16, most boys have completed puberty. If a 17- or 18-year-old still has a high-pitched voice, no facial hair, minimal body hair, and underdeveloped muscle mass, it may indicate low testosterone production. This is not a moral failing or a lifestyle issue. It is a medical signal that the hypothalamic-pituitary-gonadal axis-the hormone pathway that controls reproduction-may not be functioning properly.
A blood test measuring total testosterone, LH, and FSH can clarify what is happening. Low testosterone combined with low LH and FSH suggests a pituitary problem. Low testosterone with high LH and FSH suggests the testicles themselves are the issue.
Breast Tissue Development (Gynecomastia)
Some breast tissue in teenage boys is normal during puberty-about half of boys experience some temporary gynecomastia. But when it persists beyond puberty or develops in a young man who has already completed puberty, it can signal a hormonal imbalance. Specifically, it suggests an elevated ratio of estrogen to testosterone.
This matters for fertility because the same hormonal imbalance that causes breast tissue growth can also impair sperm production. Causes include obesity (fat tissue converts testosterone to estrogen), certain medications, marijuana use, and endocrine-disrupting chemicals in the environment.
The Lifestyle Factors That Mimic Warning Signs
Obesity and Body Composition
This is worth its own section because it is both a warning sign and a cause. A 2022 study in Obesity Reviews found that men with a BMI over 30 had a 42% higher risk of being infertile compared to men with a healthy BMI. The mechanism is clear: excess fat tissue converts testosterone to estrogen through an enzyme called aromatase. Lower testosterone means lower sperm production.
Body fat percentage matters more than weight. A young man who is "skinny fat"-normal weight but high body fat-can have the same hormonal disruption as someone who is visibly overweight. Waist circumference over 40 inches is a practical red flag.
Anabolic Steroid Use
This needs to be said directly: anabolic steroids destroy fertility. They shut down the body's natural testosterone production by suppressing LH and FSH. The testicles shrink. Sperm production stops. And the damage is not always reversible.
A 2020 study in The Journal of Clinical Endocrinology & Metabolism found that 60% of men who used anabolic steroids still had low sperm counts 12 months after stopping. Some never fully recover. If a young man is using performance-enhancing drugs, he is trading short-term muscle for long-term fertility.
Heavy Marijuana Use
The research is mixed, but the trend is clear. A 2019 systematic review in Human Reproduction Update found that men who smoked marijuana weekly had 28% lower sperm concentrations than non-users. The active compound THC binds to cannabinoid receptors in the sperm's mitochondria, impairing their ability to swim and fertilize an egg.
This does not mean occasional use is catastrophic. But daily or near-daily use in a teenage or young adult male-during the critical window when the reproductive system is still maturing-is a legitimate concern.
Tight Underwear and Heat Exposure
This one is straightforward physics. Sperm production requires a temperature about 7°F below body temperature. Tight briefs pull the testicles closer to the body, raising scrotal temperature by 1-2°F. A 2018 study in Human Reproduction found that men who wore boxers had 17% higher sperm concentrations than men who wore tight briefs.
This is not a major cause of infertility on its own, but it compounds other issues. If a young man already has a varicocele or hormonal imbalance, heat exposure from tight underwear, long bike rides, or frequent hot tub use makes everything worse.