Infertility is the inability to achieve a pregnancy after 12 months or more of regular, unprotected sex. The World Health Organization classifies it as a disease of the reproductive system, and it affects both men and women. About 8.5% of married women of reproductive age in the U.S. are infertile, and roughly 13.4% of all women ages 15 to 49 have some degree of impaired fertility.
Infertility vs. Subfertility vs. Sterility
These three terms often get mixed up, but they describe different points on a spectrum. Subfertility refers to any form of reduced fertility where conception takes longer than expected but isn’t impossible. Infertility, in stricter medical usage, sometimes overlaps with sterility, meaning the chance of a spontaneous pregnancy is close to zero. Research suggests that after about 48 months of trying without success, roughly 5% of couples reach that point of near-zero odds of conceiving on their own.
In everyday medical practice, though, “infertility” is used broadly. Your doctor will typically apply the term after 12 months of trying, regardless of whether conception might still happen eventually. It’s a clinical threshold that triggers evaluation, not necessarily a permanent diagnosis.
It’s Not Just a Female Issue
One of the most persistent misunderstandings about infertility is that it’s primarily a woman’s problem. The data tells a different story. Male factors contribute to infertility in roughly half of all cases. In about 17% of couples, both partners have contributing issues, and in nearly 14% of cases, no cause is found in either partner.
This means that when a couple is struggling to conceive, both people need to be evaluated. Starting with just one partner wastes time and can delay effective treatment.
Common Causes in Women
Ovulation disorders account for the majority of female infertility. If your body doesn’t release an egg regularly, or at all, conception can’t happen. The most common culprit is polycystic ovary syndrome (PCOS), a hormonal imbalance that disrupts ovulation and is often associated with insulin resistance, weight changes, and irregular periods.
Blocked or damaged fallopian tubes are another frequent cause. The tubes are the pathway sperm travel to reach the egg and the route a fertilized egg takes to reach the uterus. Pelvic inflammatory disease, often caused by sexually transmitted infections like chlamydia or gonorrhea, can scar and block these tubes. Previous abdominal or pelvic surgeries can also cause blockages.
Endometriosis, a condition where tissue similar to the uterine lining grows outside the uterus, affects fertility in multiple ways. It can cause scarring that physically blocks the egg and sperm from meeting, interfere with implantation of a fertilized egg, and may even damage eggs or sperm directly.
Common Causes in Men
The most common reversible cause of male infertility is a varicocele, which is a swelling of the veins that drain the testicle. Varicoceles can raise the temperature around the testicle and reduce both the number and quality of sperm. A low sperm count is defined as fewer than 15 million sperm per milliliter of semen.
Even when sperm count is normal, problems with sperm movement or function can prevent them from reaching or penetrating the egg. Hormonal imbalances, particularly low testosterone, also play a role. Infections of the reproductive tract, including some sexually transmitted infections, can interfere with sperm production or cause scarring that blocks sperm transport. Less commonly, ejaculation issues (where semen enters the bladder instead of exiting the body) or structural defects in the tubes that carry sperm contribute to the problem.
When Doctors Start Investigating
The timeline for seeking evaluation depends largely on age. For women under 35, the standard recommendation is to try for 12 months before pursuing testing. For women 35 and older, that window shortens to 6 months. Women over 40 may warrant more immediate evaluation. And if either partner has a known medical condition associated with infertility, like PCOS, endometriosis, or a history of undescended testicles, evaluation should begin right away regardless of age.
What Testing Looks Like
For men, the first and most important test is a semen analysis. You provide a semen sample, and a lab evaluates sperm count, movement, and shape. Blood tests can check hormone levels, particularly testosterone. In some cases, genetic testing or imaging of the reproductive organs with ultrasound may follow.
For women, testing typically starts with blood work to check hormone levels and confirm ovulation. An imaging test called an HSG (hysterosalpingography) is commonly used to check whether the fallopian tubes are open. During this test, a contrast dye is injected into the uterus through the cervix, and X-rays track whether the dye flows freely through the tubes. Ultrasound-based alternatives exist as well, where saline is used instead of dye to visualize the uterus. If more detail is needed, a small camera can be guided into the uterus to look for structural problems directly.
When No Cause Is Found
About 30% of infertile couples receive a diagnosis of “unexplained infertility.” This means all standard tests, including ovulation checks, fallopian tube imaging, uterine evaluation, and semen analysis, come back normal. It doesn’t mean nothing is wrong. It means current diagnostic tools can’t pinpoint the issue. Factors like egg quality, subtle hormonal timing problems, or issues with how the embryo implants may be at play but are difficult to measure with routine tests.
Unexplained infertility can be one of the more frustrating diagnoses to receive, but it doesn’t mean treatment won’t work. Many couples with unexplained infertility respond well to fertility treatments, even without a specific cause identified. The label simply reflects the limits of what testing can reveal, not the limits of what’s treatable.