Fertility can be reduced by dozens of factors, from the biological clock to everyday habits you might not think twice about. Some of these are within your control and some are not, but understanding them gives you a clearer picture of what matters most when trying to conceive.
Age Affects Both Partners
Age is the single most powerful factor in fertility, especially for women. Egg quantity and quality decline steadily from the early 30s onward, with a sharper drop after 35. By the early 40s, the probability of conceiving in any given cycle is a fraction of what it was a decade earlier. This isn’t just about eggs running out. The remaining eggs are more likely to carry chromosomal abnormalities, which raises the risk of miscarriage and makes successful implantation harder.
Men’s fertility also declines with age, though less dramatically. After 40, men are about 30% less likely to conceive within a year compared to men under 30. Sperm motility drops by roughly 0.5% to 0.8% per year, adding up to a noticeable decline over two decades. Sperm DNA damage also increases significantly after the mid-30s, with men aged 36 to 57 showing measurably higher rates of fragmented DNA than younger men. The practical result: couples where the man is 45 or older face roughly five times longer to achieve pregnancy compared to couples with a man under 25.
Body Weight and Hormonal Balance
Being significantly overweight or underweight disrupts the hormonal signals that drive ovulation and sperm production. In women, excess body fat increases estrogen levels, which can interfere with the regular hormonal cycling needed for ovulation. Being underweight can suppress those signals entirely, stopping ovulation altogether.
In men, higher body fat triggers a chain reaction. Fat tissue contains an enzyme that converts testosterone into estrogen. The more fat, the more conversion, which means lower testosterone and higher estrogen. This combination disrupts the hormonal feedback loop that controls sperm production. Lower testosterone also weakens the support cells inside the testes that physically nurture developing sperm, leading to fewer mature sperm being released. A BMI above 30 is the threshold most consistently linked to measurable declines in sperm quality, though being overweight (BMI 25 to 29.9) can also have effects.
Smoking and Alcohol
Smoking damages fertility on both sides. In women, it accelerates the loss of eggs and brings menopause earlier, while also reducing the quality of remaining eggs. In men, it impairs sperm concentration and motility. The effects are dose-dependent: heavier smokers see larger declines.
Alcohol has a clear, independent effect on conception rates. One study tracking couples trying to conceive found that women who drank no alcohol and consumed less than one cup of coffee per day conceived at a rate of about 27 pregnancies per 100 menstrual cycles. Women who drank any amount of alcohol and had more than one cup of coffee daily conceived at a rate of roughly 10.5 per 100 cycles. That’s less than half the success rate. The effect of alcohol was dose-dependent, meaning more drinks correlated with lower chances. Caffeine on its own didn’t significantly reduce fertility, but it appeared to amplify alcohol’s negative effect.
Stress and Sleep Deprivation
Chronic stress raises cortisol levels, and elevated cortisol directly interferes with the hormones that trigger ovulation. In women, high cortisol blunts the pre-ovulatory surge of luteinizing hormone (the signal that releases an egg) and lowers estrogen levels both during and after ovulation. This results in thinner uterine lining and reduced chances of implantation. Multiple studies have found that infertile women have significantly higher cortisol levels than fertile women, and in cases of unexplained infertility, cortisol is often notably elevated. In men, higher depression and anxiety scores have been linked to elevated cortisol along with lower sperm count and ejaculate volume.
Sleep deprivation compounds the problem. Testosterone levels naturally rise during sleep and peak during the first phase of deep sleep. When sleep is disrupted or cut short, testosterone drops and stress hormones rise to fill the gap. This suppresses the entire hormonal chain that drives sperm production. The effect works through the same pathway as stress: elevated stress hormones shut down reproductive hormone signaling. Getting consistently poor sleep doesn’t just make you tired; it actively lowers the hormones both men and women need to conceive.
Exercise: Too Much or Too Little
Moderate physical activity supports fertility, but intense exercise can work against it. In one study, 58% of women who ran about 32 kilometers per week showed menstrual cycle abnormalities, including skipped ovulation, compared to just 9% of sedentary women. The more vigorous the exercise, the steeper the decline: women exercising five or more hours per week at high intensity were 32% less likely to conceive than sedentary women. Women who exercised to exhaustion were 2.3 times more likely to experience fertility problems than those who took it easy.
The underlying cause is energy deficit. When you burn significantly more calories than you consume, your body interprets it as a signal that conditions aren’t right for pregnancy. The brain reduces its reproductive hormone output, leading to shorter luteal phases (the window after ovulation when implantation can occur) or skipped ovulation entirely. In one controlled study, 85% of women placed in moderate to severe caloric deficit experienced at least one cycle with a defective luteal phase. The fix is straightforward: either increasing calorie intake or dialing back from vigorous to moderate exercise often restores normal cycles.
Medical Conditions That Impair Fertility
Polycystic Ovary Syndrome (PCOS)
PCOS is one of the most common causes of female infertility. It’s diagnosed when a woman has at least two of three features: irregular or absent periods (typically fewer than eight cycles per year), elevated levels of male hormones like testosterone, and a characteristic appearance of the ovaries on ultrasound. The core fertility problem is that high testosterone disrupts the normal hormonal sequence needed to mature and release an egg. Many women with PCOS ovulate infrequently or not at all, which makes conception difficult without treatment.
Endometriosis
Endometriosis causes tissue similar to the uterine lining to grow outside the uterus, triggering chronic inflammation and, in many cases, scarring around the ovaries and fallopian tubes. This physically blocks egg release and transport, but the damage goes beyond structural problems. The inflammatory environment impairs egg quality, interferes with fertilization, and reduces the uterus’s ability to accept an embryo. Elevated oxidative stress and altered immune function in the pelvic area compound these effects.
Untreated STIs
Chlamydia and gonorrhea are particularly damaging when left untreated. About 10% to 15% of women with untreated chlamydia develop pelvic inflammatory disease, which can scar the fallopian tubes and permanently block them. Chlamydia can also cause “silent” infections in the upper reproductive tract, scarring the tubes without any noticeable symptoms. By the time a woman discovers the damage, it may already be affecting her ability to conceive.
Heat Exposure and Sperm
Sperm production requires a temperature 2 to 4 degrees Celsius below core body temperature, which is why the testes sit outside the body. Each 1°C increase in testicular temperature reduces sperm production by about 14%. That’s a steep penalty, and it doesn’t take much to trigger it. Frequent hot tub or sauna use, prolonged sitting (six or more hours in a workday), sleeping with electric blankets, and wearing tight underwear to bed have all been shown to meaningfully reduce sperm motility. Studies using scrotal warming to 40 to 43°C, roughly the temperature of a hot bath, produced significant drops in both sperm concentration and motility. The good news is that these effects are usually reversible once the heat exposure stops, though it takes about two to three months for a full cycle of new sperm to develop.
Chemical Exposures in Everyday Products
Endocrine-disrupting chemicals, particularly PFAS and phthalates, interfere with the hormonal signals that drive sperm and egg development. These compounds act on the brain-to-gonad hormone pathway, disrupting the balance needed for normal reproductive function. In men, they impair testosterone production, reduce semen quality, and increase sperm DNA fragmentation.
These chemicals are not exotic industrial pollutants. Phthalates are found in shampoo, lotion, hair spray, nail polish, fragrances, plastic food containers, and children’s toys. PFAS turn up in non-stick cookware, microwave popcorn bags, food wrappers, stain-resistant clothing, and textiles. Reducing exposure means choosing fragrance-free personal care products, avoiding plastic food storage when possible, and replacing old non-stick pans. You can’t eliminate exposure entirely, but you can meaningfully lower it.