Fertility drops when any step in the reproductive process is disrupted, from egg and sperm quality to ovulation, fertilization, and implantation. Some causes are biological and unavoidable, like aging. Others are lifestyle factors you can change. Here’s what the evidence shows about the most significant fertility reducers for both women and men.
Age Is the Single Biggest Factor
A woman’s peak reproductive years fall between the late teens and late 20s. By age 30, fertility begins declining, and the drop accelerates after the mid-30s. In practical terms, a healthy couple in their 20s has roughly a 1-in-4 chance of conceiving in any given menstrual cycle. By age 40, that falls to about 1 in 10. By 45, natural conception is unlikely.
Two things happen simultaneously as women age. The total number of eggs decreases, and the remaining eggs are more likely to carry abnormal chromosomes. This raises both the difficulty of getting pregnant and the risk of chromosomal conditions. The chance of a pregnancy affected by Down syndrome, for example, goes from about 1 in 1,250 at age 20 to 1 in 86 at age 40.
Men’s fertility also declines with age, though more gradually. Sperm quality, including motility and DNA integrity, worsens over time, and older paternal age is linked to longer time to conception and higher miscarriage rates.
Body Weight and Ovulation
Being significantly overweight disrupts the hormonal signals that trigger ovulation. Women with a BMI above 27 have roughly 2.4 times the risk of anovulatory infertility (meaning they stop ovulating regularly) compared to women at a normal weight. That risk climbs further as BMI increases: women with a BMI over 32 face nearly triple the risk. The pattern holds even when researchers account for age and exercise habits.
Excess weight also reduces the effectiveness of fertility treatments. Higher BMI correlates with lower implantation rates, lower clinical pregnancy rates, and lower live birth rates during IVF. Abdominal fat specifically appears to reduce the odds of ovulating in response to common fertility medications.
Being significantly underweight matters too. Very low body fat can shut down the hormonal cycle entirely, causing periods to stop and ovulation to cease. This is common in women with eating disorders or those who exercise at extremely high intensities without adequate nutrition.
Smoking and Alcohol
Smoking damages fertility on both sides. In men, even moderate smoking (up to 10 cigarettes a day) is associated with lower sperm counts. Heavier smoking, up to two packs a day or more, compounds the damage. In women, smoking accelerates egg loss and is linked to earlier menopause, effectively shortening the window of fertility by several years.
Alcohol’s effects depend on quantity. In men, regularly drinking more than two drinks a day is associated with reduced sperm concentration and quality. Getting intoxicated at least once a month adds further risk. For women, the threshold for reproductive harm appears to be lower. Research has used roughly one drink per day as the cutoff for medium health risk in women, compared to two drinks per day for men. Heavy or binge drinking disrupts the menstrual cycle, interferes with ovulation, and increases miscarriage risk.
PCOS and Hormonal Disorders
Polycystic ovary syndrome is one of the most common causes of female infertility worldwide. It’s characterized by excess androgens (male hormones), irregular or absent ovulation, and cysts on the ovaries. Global data shows that PCOS-attributable infertility affects roughly 638 per 100,000 women, making it a far larger contributor to infertility than endometriosis in most regions.
PCOS doesn’t mean you can’t get pregnant, but it often means you ovulate unpredictably or not at all. The condition is closely tied to insulin resistance, which is why weight management and blood sugar control often improve ovulation in affected women. Other hormonal conditions that reduce fertility include thyroid disorders (both overactive and underactive), elevated prolactin levels, and premature ovarian insufficiency, where the ovaries stop working normally before age 40.
Endometriosis and Structural Problems
Endometriosis occurs when tissue similar to the uterine lining grows outside the uterus, commonly on the ovaries, fallopian tubes, and pelvic lining. It reduces fertility through structural changes (scar tissue can block or distort the fallopian tubes), disruption of the environment where fertilization happens, and inflammation that impairs egg quality. Its overall contribution to infertility at the population level is lower than PCOS, partly because surgical and assisted reproductive treatments can be effective for many women with the condition.
Other structural issues that decrease fertility include blocked fallopian tubes (often from prior infections or surgery), uterine fibroids that distort the uterine cavity, and uterine polyps that interfere with implantation.
Heat Exposure and Male Fertility
Sperm production requires temperatures slightly below core body temperature, which is why the testes sit outside the body. Anything that raises scrotal temperature for extended periods can significantly impair sperm quality. A meta-analysis of multiple studies found that heat exposure reduces sperm count, concentration, motility, and the percentage of normally shaped sperm.
The practical sources of harmful heat are specific and worth knowing. Regular sauna use causes measurable sperm impairment, including damage to sperm DNA and mitochondrial function. Sitting for six or more hours during a typical workday reduces sperm motility. Wearing tight-fitting underwear while sleeping, using electric blankets, and working in hot environments all contribute. The good news is that sperm damage from heat exposure is often reversible once the source is removed, though recovery takes roughly two to three months because that’s how long new sperm take to mature.
Everyday Chemicals
Phthalates, chemicals found in plastics, personal care products, food packaging, and vinyl flooring, are among the most studied fertility-disrupting compounds. In men attending infertility clinics, higher levels of phthalate byproducts in urine and semen have been linked to decreased sperm concentration, reduced motility, DNA damage in sperm, and abnormal sperm shape. These findings have been replicated across studies in Chinese, Polish, and other populations.
Phthalates mimic or interfere with hormones. In animal studies, prenatal exposure causes malformed reproductive tissue in male offspring, including damaged sperm-producing structures in the testes. While human exposure levels are far lower than those used in animal experiments, the consistency of findings across species and study designs has made these chemicals a growing concern. Reducing exposure means choosing fragrance-free personal care products, avoiding microwaving food in plastic containers, and limiting contact with soft vinyl products.
Medications That Interfere With Ovulation
Common painkillers can quietly suppress fertility. Non-steroidal anti-inflammatory drugs like ibuprofen and naproxen inhibit ovulation across all mammalian species studied. They work by blocking an enzyme essential to the production of prostaglandins, which are needed for the follicle to release an egg. Occasional use is unlikely to cause problems, but regular daily use during the fertile window may be an overlooked cause of difficulty conceiving.
Other medications that can reduce fertility include certain antidepressants (which may raise prolactin levels and disrupt ovulation), anabolic steroids (which shut down natural sperm production in men, sometimes for months after stopping), and some blood pressure medications. Chemotherapy and radiation therapy can cause permanent damage to eggs or sperm-producing cells depending on the type and dose.
Sexually Transmitted Infections
Chlamydia and gonorrhea are the two STIs most directly linked to infertility. When left untreated in women, either can travel upward from the cervix into the uterus and fallopian tubes, causing pelvic inflammatory disease. This leads to scarring and blockage of the tubes, preventing the egg from reaching the uterus. The damage is often silent, with many women unaware they had an infection until they have trouble conceiving years later. In men, untreated infections can cause inflammation of the reproductive tract that impairs sperm transport.
Stress, Sleep, and Exercise Extremes
Chronic psychological stress can suppress the hormonal signals that drive ovulation. The mechanism is straightforward: the stress hormone cortisol, when elevated over long periods, interferes with the release of reproductive hormones from the brain. Studies on women undergoing IVF have found that higher stress levels correlate with lower pregnancy rates, though separating cause from effect in this area is difficult.
Sleep disruption, particularly shift work or consistently getting fewer than six hours per night, is associated with menstrual irregularity and longer time to conception. Extreme exercise without adequate calorie intake suppresses reproductive function in both sexes. In women, this manifests as lost periods. In men, intense endurance training can temporarily lower testosterone and sperm counts. Moderate exercise, on the other hand, generally supports fertility by improving insulin sensitivity, reducing inflammation, and maintaining a healthy weight.