Infertility has dozens of possible causes, spanning hormonal imbalances, structural problems, lifestyle factors, and environmental exposures, and it affects both partners roughly equally. Clinically, infertility is evaluated after 12 months of regular unprotected intercourse for women under 35, or after 6 months for women 35 and older. About a third of cases trace to a female factor, a third to a male factor, and the remainder to a combination of both or no identifiable cause at all.
Ovulation Problems
Irregular or absent ovulation is one of the most common reasons women struggle to conceive. Without the release of an egg each cycle, fertilization simply can’t happen. The most frequent culprit is polycystic ovary syndrome (PCOS), which affects 10 to 13% of women of reproductive age. PCOS involves elevated levels of androgens (male-type hormones like testosterone), which interfere with the normal hormonal signals that trigger ovulation. Many women with PCOS also have insulin resistance, which further disrupts the hormonal environment needed for regular cycles.
Other ovulation disruptors include thyroid disorders, excess prolactin (a hormone normally involved in milk production), and diminished ovarian reserve, where the number or quality of remaining eggs drops earlier than expected. Extreme stress, very low body weight, and excessive exercise can also shut down ovulation by signaling to the brain that the body isn’t in a state to support pregnancy.
Fallopian Tube Damage
The fallopian tubes are where egg and sperm meet, so any blockage or scarring there can prevent conception entirely. Pelvic inflammatory disease (PID), usually caused by sexually transmitted infections like chlamydia or gonorrhea, is the leading cause of tubal damage. About 12% of women suffer enough tubal scarring from a single episode of PID to become infertile. After three episodes, that rate climbs to 50%.
PID often causes subtle or no symptoms, which means damage can accumulate before a woman even knows there’s a problem. Previous abdominal or pelvic surgery, a ruptured appendix, or ectopic pregnancy can also leave scar tissue that blocks or narrows the tubes.
Endometriosis
Endometriosis occurs when tissue similar to the uterine lining grows outside the uterus, commonly on the ovaries, fallopian tubes, and pelvic lining. It causes infertility through several routes at once. Adhesions and scar tissue can physically distort the pelvis, blocking the fallopian tubes or preventing the ovary from releasing an egg into the tube. When endometriosis forms cysts on the ovaries (called endometriomas), the surrounding ovarian tissue shows lower follicle density, increased scarring, and loss of the specialized tissue that supports egg development.
The inflammation itself is also damaging. Endometriosis creates a chronic inflammatory environment that generates oxidative stress in ovarian tissue, harming cells and reducing their ability to function. Even mild endometriosis that doesn’t cause visible structural damage can impair fertility through these inflammatory pathways, which is why some women with minimal disease still have difficulty conceiving.
Uterine and Cervical Factors
Problems with the uterus itself can prevent a fertilized egg from implanting or a pregnancy from developing. Fibroids, particularly those that grow into the uterine cavity, can distort the lining and interfere with implantation. Uterine polyps have a similar effect. Scarring inside the uterus from previous surgeries or infections (a condition called Asherman syndrome) can reduce the usable surface area for implantation.
Structural abnormalities present from birth, such as a uterine septum (a wall of tissue dividing the cavity), can also contribute. Cervical factors are less common but include scarring from procedures or insufficient cervical mucus, which sperm need to travel through.
Male Factor Infertility
Male factors contribute to roughly half of all infertility cases, either alone or alongside a female factor. A standard semen analysis looks at three key measures: sperm concentration (at least 15 million per milliliter is considered normal), motility (at least 40% of sperm should be moving), and morphology (at least 4% should have a normal shape). Falling below these thresholds doesn’t guarantee infertility, but it significantly reduces the odds of natural conception.
Varicoceles, which are enlarged veins in the scrotum that raise testicular temperature, are the most common identifiable structural cause. They’re found in about 35% of men with primary infertility (meaning they’ve never fathered a child) and in 50 to 80% of men with secondary infertility (meaning they conceived before but now can’t). Other causes include undescended testicles, blockages in the reproductive tract, hormonal imbalances, genetic conditions, and prior infections or injuries.
Some medications, including testosterone replacement therapy and certain antidepressants, can suppress sperm production. This is a frequently overlooked cause, especially since testosterone supplementation is increasingly common among younger men.
Age and Ovarian Reserve
Age is the single strongest predictor of female fertility. Women are born with all the eggs they’ll ever have, and both the number and genetic quality of those eggs decline steadily over time. The decline accelerates after age 35 and becomes steep after 40. By 40, the chance of conceiving naturally in any given cycle drops to about 5%, and the risk of miscarriage rises sharply because a higher proportion of eggs carry chromosomal abnormalities.
Male fertility also declines with age, though more gradually. Sperm quality, including DNA integrity, decreases after age 40, and older paternal age is linked to longer time-to-conception and higher miscarriage rates. The effect is less dramatic than the female age factor, but it’s real and often underestimated.
Weight and Lifestyle Factors
Body weight has a well-documented effect on fertility for both sexes. In women, a BMI above 27 more than doubles the risk of anovulatory infertility compared to women at a normal weight, and the risk continues climbing with each additional BMI point. The mechanism is largely hormonal: excess fat tissue produces estrogen and promotes insulin resistance, both of which disrupt the signals needed for regular ovulation. Being significantly underweight can shut down ovulation altogether.
In men, obesity is associated with lower sperm counts and reduced sperm motility, though the relationship is less consistent across studies than it is for women. Excess body fat increases the conversion of testosterone to estrogen, which can suppress the hormonal signals driving sperm production.
Smoking reduces fertility in both sexes, accelerating egg loss in women and damaging sperm DNA in men. Heavy alcohol use impairs ovulation and lowers testosterone. Even moderate chronic stress can suppress reproductive hormones, though its independent contribution to infertility is harder to quantify.
Environmental Chemical Exposures
A growing body of evidence links everyday chemical exposures to reproductive harm. Endocrine-disrupting chemicals, found in plastics, pesticides, food packaging, and personal care products, can mimic or block the body’s hormones. The most studied include BPA (common in plastics and can linings), phthalates (found in fragrances, vinyl, and food packaging), and certain pesticides.
These chemicals interfere with fertility in several ways. They can bind to hormone receptors and trigger false signals, alter the timing of the hormonal surge that triggers ovulation, and create oxidative stress in ovarian tissue that damages eggs. BPA specifically has been shown to impair egg development and reduce egg quality by altering how genes are turned on and off. Some of these chemical effects can even carry over to the next generation: prenatal exposure to certain compounds can cause reproductive changes in offspring through modifications to DNA that don’t alter the genetic code itself but change how it’s read.
Minimizing exposure is difficult since these chemicals are widespread, but reducing use of plastic food containers (especially when heated), choosing fragrance-free products, and washing produce can lower your intake.
Unexplained Infertility
After a complete workup including ovulation testing, imaging of the uterus and fallopian tubes, and semen analysis, 15 to 30% of couples receive a diagnosis of unexplained infertility. This doesn’t mean nothing is wrong. It means the standard tests aren’t sensitive enough to detect the problem. Subtle issues with egg quality, sperm function, fertilization, embryo development, or implantation can all fall below the radar of current diagnostic tools.
Couples with unexplained infertility generally have a better prognosis than those with identified causes. Many conceive with relatively simple interventions like ovulation-stimulating medication combined with intrauterine insemination, and a meaningful percentage will conceive on their own given additional time. For those who don’t, IVF remains effective because it bypasses many of the hidden steps where things might be going wrong.