Infertility has no single cause. It stems from a range of conditions affecting ovulation, sperm production, reproductive anatomy, or some combination of all three. Roughly one-third of infertility cases trace back to female factors, one-third to male factors, and the remaining third to a mix of both or no identifiable cause at all. Understanding what can go wrong, and why, is the first step toward knowing what to do about it.
Ovulation Problems
The most common cause of female infertility is irregular or absent ovulation. Without a mature egg released each cycle, conception can’t happen. Polycystic ovary syndrome (PCOS) is the leading culprit. In women with PCOS, the ovaries develop many small follicles that stall before any single one matures enough to release an egg. This failure of “dominance,” where one follicle outgrows the rest, means ovulation either happens unpredictably or not at all.
The hormonal picture behind PCOS involves several overlapping problems: excess production of a hormone called LH, elevated levels of male hormones (androgens), and high insulin. These factors reinforce each other. Elevated insulin drives the ovaries to produce more androgens, which further disrupts follicle development. The result is cycles that are long, irregular, or missing entirely. PCOS affects an estimated 6 to 12 percent of women of reproductive age, making it one of the most widespread fertility barriers.
Other ovulation disruptors include thyroid disorders, excess prolactin (a hormone normally involved in milk production), and premature ovarian insufficiency, where the ovaries stop functioning normally before age 40.
Structural and Anatomical Causes
Even when ovulation happens normally, physical blockages or damage to the reproductive tract can prevent egg and sperm from meeting. Endometriosis is one of the most significant structural causes. Tissue similar to the uterine lining grows outside the uterus, often on the ovaries, fallopian tubes, or pelvic lining. Between 25 and 50 percent of women with infertility have endometriosis, and roughly 30 to 50 percent of women with endometriosis experience infertility.
In more advanced stages, endometriosis creates scar tissue and adhesions that physically distort the pelvic anatomy. Fallopian tubes can become blocked or kinked, the finger-like fimbriae at the end of each tube lose their ability to sweep up a released egg, and ovarian cysts filled with old blood (endometriomas) can damage surrounding egg-containing tissue. Even in milder cases, endometriosis appears to create an inflammatory environment that can impair egg quality and embryo implantation.
Blocked fallopian tubes from causes other than endometriosis, such as previous pelvic infections or prior surgery, account for another significant share of structural infertility. Uterine fibroids, polyps, or an unusually shaped uterus can also interfere with implantation or pregnancy maintenance.
Male Factor Infertility
Male factors contribute to nearly half of all infertility cases, yet they’re often overlooked in early conversations. The most common issue is low sperm count or poor sperm quality, which can result from a variety of conditions.
Varicoceles, enlarged veins in the scrotum, are the most frequently identified cause of male infertility. They’re found in roughly 40 percent of men being evaluated for fertility problems. The mechanism involves heat: varicoceles allow warm blood from the abdomen to pool around the testes, raising scrotal temperature above the narrow range sperm production requires. This excess heat damages DNA and proteins within the cells that produce sperm, triggers higher rates of sperm cell death, and reduces the overall count. The sperm that do survive often have reduced motility, partly because of increased levels of reactive oxygen species (a form of cellular stress) and the development of antibodies that attack sperm.
Other male causes include hormonal imbalances, undescended testes, genetic conditions like Klinefelter syndrome, prior infections, and blockages in the ducts that carry sperm from the testes to the ejaculate.
Age and Egg Supply
Age is the single most predictable factor in female fertility decline. A healthy 30-year-old woman has about a 20 percent chance of conceiving in any given cycle. Fertility begins to drop more noticeably after 35, and by 40, the chance per cycle falls below 5 percent. By 43, even IVF success rates drop under 5 percent, and by 45, using donor eggs becomes the only realistic assisted-reproduction option for most women.
This decline isn’t just about having fewer eggs. The eggs that remain are more likely to carry chromosomal abnormalities, which raises miscarriage risk and lowers the odds that a fertilized egg will develop normally. Women are born with all the eggs they’ll ever have, roughly one to two million at birth, declining to about 300,000 by puberty. There’s no way to produce new ones.
Male fertility also declines with age, though more gradually. Sperm quality, including DNA integrity and motility, decreases after 40, and it can take longer to conceive with an older male partner.
Body Weight and Metabolic Health
Weight significantly affects fertility in both sexes. In women, a BMI above 27 roughly doubles to triples the risk of anovulatory infertility compared to women at a normal weight. The relationship is dose-dependent: the higher the BMI at age 18, the greater the risk of ovulation problems later. At a BMI of 24 to 26, the relative risk is about 1.3 times normal. At a BMI above 32, it jumps to 2.7 times normal. Excess body fat increases insulin resistance and circulating estrogen levels, both of which disrupt the hormonal signaling that triggers ovulation.
Being significantly underweight causes problems too. Very low body fat can shut down the hormonal cascade needed for ovulation entirely, a condition sometimes seen in athletes or people with eating disorders.
In men, higher BMI is consistently linked to lower testosterone levels. Excess fat tissue converts testosterone into estrogen, shifting the hormonal balance away from what’s needed for healthy sperm production. Weight loss in overweight men has been shown to improve sperm parameters.
Lifestyle and Environmental Factors
Smoking reduces fertility in both men and women. In women, chemicals in cigarette smoke accelerate egg loss and can bring on menopause one to four years earlier than expected. In men, smoking lowers sperm count and increases the percentage of abnormally shaped sperm.
Heavy alcohol use disrupts ovulation in women and lowers testosterone in men. Even moderate drinking (more than a few drinks per week) has been associated with longer time to conception in some studies, though the exact threshold varies.
Chronic stress doesn’t directly destroy eggs or sperm, but it can suppress the hormonal signals that drive ovulation and may reduce sexual frequency, which matters when timing is everything. Exposure to certain environmental chemicals, including some pesticides, industrial solvents, and plasticizers like BPA, has been linked to reduced sperm quality and disrupted menstrual cycles, though pinpointing individual exposures is difficult.
Unexplained Infertility
In about 10 to 15 percent of couples, standard testing reveals no clear cause. Ovulation is happening, tubes are open, sperm looks normal, and yet pregnancy doesn’t occur. This doesn’t mean nothing is wrong. It means current diagnostic tools can’t identify the specific problem. Possible hidden factors include subtle egg quality issues, problems with embryo implantation, or sperm dysfunction that doesn’t show up on a standard semen analysis. Many couples with unexplained infertility do eventually conceive, either on their own or with treatment, but the diagnosis can be frustrating precisely because there’s no obvious target to address.