Infertility affects roughly 1 in 6 adults worldwide, and its causes span a wide range of factors in both men and women. About 17.5% of the adult population experiences infertility at some point, with similar rates in high-income and low-income countries. The condition is clinically defined as the inability to conceive after 12 months of regular unprotected sex, or after 6 months for women 35 and older.
There is rarely a single, simple explanation. Infertility can stem from problems with ovulation, sperm production, structural issues in the reproductive tract, hormonal imbalances, lifestyle factors, or some combination. In about 30% of cases, no identifiable cause is found at all.
How Male Factors Contribute
The male partner is involved in 40% to 50% of all infertility cases, making it roughly as common as female-factor infertility. The most frequent type is idiopathic male infertility, meaning one or more semen parameters (count, motility, or shape) are abnormal, but no clear underlying reason can be identified.
One of the most recognized physical causes is a varicocele, an enlargement of veins within the scrotum that raises testicular temperature and disrupts sperm production. A large European study found varicoceles in about 16.6% of men referred for infertility evaluation. The damage happens gradually: heat, reduced blood flow, and oxidative stress first impair sperm motility, then over time can reduce sperm concentration. Oxidative stress from a varicocele also fragments sperm DNA, lowering overall sperm quality. In men with varicoceles over age 30, testosterone levels tend to drop more than expected, likely because the heat damages the cells responsible for producing the hormone.
Other male causes include hormonal imbalances, genetic conditions, undescended testicles, infections that scar the reproductive tract, and certain medications. But in many cases, lifestyle is a significant contributor, which is covered further below.
Ovulation Disorders
For women, the most common category of infertility involves problems with ovulation. If an egg isn’t released on a regular schedule, conception becomes difficult or impossible. Polycystic ovary syndrome (PCOS) is the leading culprit here, and around 70% of women with PCOS experience infertility.
PCOS disrupts ovulation through a cascade of hormonal problems. About 70% of women with the condition have insulin resistance, meaning their bodies produce excess insulin to compensate. That extra insulin stimulates the ovaries to produce too many androgens (male-type hormones). At the same time, the balance between two key reproductive hormones shifts: levels of luteinizing hormone climb while follicle-stimulating hormone stays relatively low. Without enough follicle-stimulating hormone, the ovaries can’t mature eggs properly. The result is a self-perpetuating cycle where hormones remain stuck in a non-ovulatory pattern.
Other ovulation disorders include thyroid problems, excess prolactin production, and premature ovarian insufficiency, where the ovaries stop functioning normally before age 40.
Structural Problems in the Pelvis
Even when ovulation works normally, physical blockages or damage in the reproductive tract can prevent pregnancy. Two major causes stand out: endometriosis and pelvic inflammatory disease.
Endometriosis occurs when tissue similar to the uterine lining grows outside the uterus, often on the ovaries, fallopian tubes, or pelvic walls. In its more advanced stages, it creates adhesions and scarring that distort the normal anatomy of the pelvis. These adhesions can block the fallopian tubes, restrict the movement of the finger-like structures (fimbriae) that guide eggs into the tubes, and physically prevent sperm and egg from meeting. Even without visible blockages, endometriosis produces inflammatory substances like prostaglandins and cytokines that can impair ovarian function, fertilization, and implantation.
Pelvic inflammatory disease, usually caused by sexually transmitted infections like chlamydia or gonorrhea, can scar and damage the fallopian tubes from the inside. The degree of tubal damage correlates strongly with the extent of pelvic adhesions when PID is the underlying cause.
Uterine Fibroids and Polyps
Fibroids are noncancerous growths in the uterine wall, diagnosed in about 5% to 10% of women with infertility. They are the sole cause of infertility in roughly 2% to 3% of those cases. The closer a fibroid sits to the uterine cavity and its lining, the more it interferes with conception.
Fibroids can reduce fertility in several ways. Large ones may compress the opening of the fallopian tubes, physically blocking sperm or embryo transport. Those that bulge into the uterine cavity distort its shape, making implantation harder. Women with cavity-distorting fibroids consistently show lower implantation and pregnancy rates and higher miscarriage rates. Fibroids can also disrupt the rhythmic contractions of the uterine wall that help move sperm and embryos into position, alter blood supply to the lining, change the local immune environment, and even shift the balance of bacteria in the uterus.
Age and Egg Quality
A woman’s age is one of the strongest predictors of fertility, and its effects go beyond simply having fewer eggs. At birth, most girls have about 2 million eggs. By adolescence, that number drops to around 400,000. By age 37, roughly 25,000 remain. By menopause, around age 51, only about 1,000 immature eggs are left, and they are no longer fertile.
But the decline isn’t just about quantity. As eggs age, they accumulate chromosomal errors during cell division. This means that even when ovulation occurs normally, an older egg is more likely to produce an embryo with the wrong number of chromosomes, leading either to failed implantation, early miscarriage, or genetic conditions. This is why fertility guidelines recommend evaluation after just 6 months of trying for women 35 and older, rather than the standard 12 months for younger women.
Lifestyle and Environmental Risks
Modifiable lifestyle factors play a real role in both male and female fertility. Being overweight or obese is linked to infertility in both sexes. In men, excess body weight is associated with metabolic syndrome, a cluster of conditions including high blood sugar, abnormal cholesterol, and excess abdominal fat. One study found metabolic syndrome in nearly 18% of infertile men compared to just 6% of fertile men.
Smoking independently damages reproductive function. In the same study, 29% of infertile men were smokers versus 14% of fertile men. Smoking harms sperm quality, reduces egg reserves in women, and increases the risk of miscarriage. Excessive alcohol consumption adds further strain on male fertility by disrupting hormone production and sperm development. Poor diet and lack of physical activity compound these effects, particularly when they contribute to insulin resistance or chronic inflammation.
Unexplained Infertility
Even after thorough testing, about 30% of infertile couples receive a diagnosis of unexplained infertility, meaning all standard evaluations come back normal. This doesn’t mean nothing is wrong. It means current diagnostic tools aren’t sensitive enough to identify the problem. Potential hidden causes include subtle defects in egg or sperm quality that don’t show up on standard tests, problems with fallopian tube function that imaging can’t detect, poor endometrial receptivity (where the uterine lining doesn’t support implantation despite looking normal), and early-stage endometriosis that hasn’t yet caused visible damage.
How Infertility Is Diagnosed
A standard infertility workup evaluates both partners. For the male partner, the first step is a semen analysis, which measures sperm count, motility, and shape. He should avoid ejaculation for 48 hours but no more than six days before the test.
For women, the evaluation typically starts with blood tests on day three of the menstrual cycle to measure hormone levels that reflect ovarian reserve and function. A progesterone test later in the cycle can confirm whether ovulation actually occurred. Over-the-counter urine ovulation kits can also help track whether and when ovulation is happening at home.
If these baseline tests don’t reveal the problem, imaging and procedures come next. A hysterosalpingogram (HSG) uses X-ray and injected dye to check whether the fallopian tubes are open and to examine the shape of the uterine cavity. Pelvic ultrasound can identify fibroids or ovarian cysts. If endometriosis or internal adhesions are suspected, a laparoscopy allows a surgeon to look directly inside the pelvic cavity through a small incision. A hysteroscopy, where a thin camera is passed through the cervix, gives a close-up view of the inside of the uterus to check for polyps, scar tissue, or structural abnormalities.