Difficulty getting pregnant affects roughly 1 in 6 couples, and the causes split more evenly than most people expect. About 50% of infertility cases trace to a female factor alone, 20 to 30% to a male factor alone, and the remaining 20 to 30% to a combination of both partners. In about 30% of cases, no clear cause is ever found. If you’ve been trying for a year without success (or six months if you’re 35 or older), there’s a medical framework for figuring out what’s going on.
How Long Is Too Long to Keep Trying
The general guideline is 12 months of regular, unprotected sex without conception before doctors consider a fertility evaluation, assuming the female partner is under 35. That threshold drops to 6 months at age 35 and older, because egg quantity and quality decline more steeply after that point. These timelines aren’t arbitrary cutoffs. They reflect the reality that even healthy couples with no fertility issues have only about a 20 to 25% chance of conceiving in any given cycle. It takes time, and sometimes perfectly normal biology just needs more cycles to line up.
Ovulation Problems
The most common female fertility issue is irregular or absent ovulation. Without a released egg, conception can’t happen. Polycystic ovary syndrome (PCOS) is the leading cause, affecting how often and whether your ovaries release eggs. The hallmark signs are irregular periods (cycles longer than 35 days or fewer than 8 periods a year), excess body or facial hair from elevated male hormones, and a characteristic appearance of the ovaries on ultrasound. You don’t need all three signs for a diagnosis; any two of those three is enough.
Other conditions that disrupt ovulation include thyroid disorders, elevated levels of the hormone prolactin, and primary ovarian insufficiency, where the ovaries stop functioning normally before age 40. Regular monthly periods are generally a good sign that ovulation is happening, but not a guarantee. Some women ovulate inconsistently even with seemingly normal cycles.
Structural and Anatomical Causes
Blocked or damaged fallopian tubes prevent sperm from reaching the egg, or stop a fertilized egg from traveling to the uterus. Pelvic inflammatory disease (PID), usually caused by untreated sexually transmitted infections, is a major culprit. About 1 in 8 people who’ve had PID have difficulty getting pregnant afterward, and up to 10% become permanently infertile because scar tissue seals off the tubes.
Endometriosis is another significant structural cause. Tissue similar to the uterine lining grows outside the uterus, triggering chronic inflammation that creates a hostile environment for eggs, sperm, and embryos. The inflammation impairs how the fallopian tubes move, blocks sperm from reaching the egg, and can trap eggs behind pelvic adhesions. Even mild endometriosis can reduce fertility, though the effect tends to worsen with more advanced disease.
Uterine abnormalities also play a role. Fibroids, polyps, scar tissue inside the uterine cavity, and variations in uterine shape (such as a uterus divided by a wall of tissue) can all interfere with implantation or increase miscarriage risk.
Male Factor Infertility
Because male factors contribute to 20 to 30% of cases on their own and overlap with female factors in another 20 to 30%, a semen analysis is one of the first tests in any fertility workup. The test measures sperm count, movement, and shape. Healthy reference values include at least 15 million sperm per milliliter, at least 40% of sperm showing movement, and at least 4% with normal shape.
Low numbers in any of those categories can reduce the odds of natural conception, though they don’t make it impossible. Common causes of poor sperm quality include varicoceles (enlarged veins in the scrotum that raise testicular temperature), hormonal imbalances, prior infections, certain medications, and genetic factors. Heat exposure, heavy alcohol use, smoking, and anabolic steroid use also suppress sperm production. Unlike egg supply, sperm production is continuous, so improvements in lifestyle or treatment of an underlying condition can sometimes restore normal levels within a few months.
Weight and Lifestyle Factors
Body weight has a measurable impact on fertility for both partners. In women, a BMI above 27 roughly doubles the risk of ovulation-related infertility compared to women in the normal range (18.5 to 24.9). That risk climbs further as BMI increases. Even among women who still get regular periods, hormone profiles shift with higher body weight, suggesting that the disruption operates on a spectrum rather than flipping a single switch. Being significantly underweight (BMI below 18.5) also disrupts hormonal signaling and can shut down ovulation entirely.
In men, excess weight is linked to lower sperm counts and reduced sperm quality, partly because fat tissue converts testosterone into estrogen. Smoking, heavy drinking, recreational drug use, and chronic stress affect fertility in both sexes. These factors rarely explain infertility on their own, but they can tip the balance when other mild issues are already present.
Age and Egg Supply
Female fertility peaks in the early to mid-20s and begins a gradual decline around age 30 that accelerates after 35. This isn’t just about egg quantity. Egg quality, meaning the likelihood that an egg will have the correct number of chromosomes, also drops with age. That translates to lower conception rates per cycle and higher miscarriage rates. By 40, the chance of conceiving naturally in a given month is roughly 5%, compared to 20 to 25% in the late 20s.
Male fertility also declines with age, though more gradually. Sperm count, motility, and DNA integrity all decrease over time. Men over 40 take longer to conceive with their partners and have slightly higher rates of miscarriage and certain genetic conditions in offspring.
Unexplained Infertility
About 30% of couples complete a full workup and hear that everything looks normal. This diagnosis, called unexplained infertility, means that standard tests haven’t identified the problem, not that nothing is wrong. To reach this label, doctors confirm that ovulation is happening, fallopian tubes are open, the uterus looks normal, and the semen analysis falls within reference ranges. The couple also needs to be having sex frequently enough and the female partner is typically 40 or younger.
Possible explanations that current testing can’t easily detect include subtle egg quality issues, problems with how the embryo implants, or immune factors in the uterine lining. It’s a frustrating diagnosis, but couples with unexplained infertility often still respond well to fertility treatments because there’s no major structural barrier in the way.
What Testing Looks Like
A fertility evaluation typically starts with blood tests and imaging for the female partner and a semen analysis for the male partner. Blood tests check hormone levels that reflect ovarian reserve (how many eggs remain), whether ovulation is occurring, and thyroid function. One key blood test measures anti-Müllerian hormone (AMH), which indicates how many eggs your ovaries have left. A high AMH level suggests a larger remaining supply; a low level means the window may be shorter. AMH is especially useful for women under 40 who show early signs of declining fertility.
To check for structural problems, doctors often use a hysterosalpingogram (HSG). During this X-ray procedure, dye is injected through the cervix into the uterus and fallopian tubes. If the dye flows freely through both tubes and spills out the ends, the tubes are open. If the dye stops at a certain point, there’s a blockage. The same test can reveal an irregularly shaped uterus, fibroids, polyps, or scar tissue inside the uterine cavity.
For the male partner, a semen analysis is straightforward and noninvasive. If results come back abnormal, follow-up may include hormone testing or an ultrasound of the reproductive tract. In many cases, the combination of blood work, an HSG, and a semen analysis is enough to identify or rule out the most common causes, giving you and your doctor a clearer picture of what’s happening and what options make sense next.