Roughly 1 in 6 adults worldwide experience infertility at some point, so if you’re struggling to conceive, you’re far from alone. The World Health Organization puts the global prevalence at about 17.5%, with similar rates across high-income and lower-income countries. The reasons range from hormonal imbalances and structural problems to sperm quality, lifestyle factors, and environmental exposures. In about 30% of cases, no clear cause is ever found. Understanding the most common reasons can help you figure out what to ask about and what steps to take next.
How Long Is Too Long?
The clinical threshold depends on your age. If you’re under 35, providers typically won’t diagnose infertility until you’ve been having regular, unprotected sex for 12 months without a pregnancy. If you’re 35 or older, that window shortens to six months. These timelines exist because conception is a numbers game even for perfectly healthy couples. In any given cycle, the odds of pregnancy are only about 15 to 25 percent, so it can legitimately take several months of well-timed attempts.
That said, the cutoffs aren’t meant to stop you from seeking help sooner. If you have irregular periods, a known condition like endometriosis, or a partner with a prior fertility concern, there’s no reason to wait out the full timeline before talking to a specialist.
Ovulation Problems
The most common female factor is irregular or absent ovulation. Without a mature egg being released each cycle, conception can’t happen. Polycystic ovary syndrome (PCOS) is the leading cause. In PCOS, the balance between two key brain-signaling hormones gets disrupted: the hormone that triggers egg maturation (FSH) stays too low, while the hormone that normally surges to release the egg (LH) stays too high. At the same time, androgen levels rise. The combined effect is that follicles in the ovary start developing but stall before they mature, so eggs either aren’t released at all or are released unpredictably.
PCOS isn’t the only culprit. Thyroid disorders, elevated prolactin levels, and problems with the hypothalamus (the part of the brain that coordinates your reproductive hormones) can all interfere with ovulation. Extremely low body weight or intense endurance exercise can suppress ovulation entirely by signaling to the brain that the body isn’t in a state to support pregnancy.
Fallopian Tube and Uterine Issues
Even with perfectly timed ovulation, a blockage or structural problem between the ovary and the uterus can prevent the egg and sperm from meeting. The fallopian tubes are especially vulnerable. Pelvic inflammatory disease, often caused by untreated sexually transmitted infections, is a well-known cause of tubal scarring. Endometriosis is another. Endometrial tissue can grow into the tube’s inner channel and cause direct obstruction, or repeated cycles of bleeding and inflammation on the tube’s outer surface can lead to scarring, kinking, and fluid buildup (called a hydrosalpinx) that blocks the passage entirely.
Inside the uterus, fibroids, polyps, or scar tissue from prior surgeries can make it harder for an embryo to implant. These problems don’t always cause noticeable symptoms, which is why imaging tests are a routine part of a fertility workup.
Sperm Quality Matters Just as Much
Male factors contribute to infertility in a significant number of couples, affecting roughly 1 in 10 men. A semen analysis measures three core parameters: concentration (at least 20 million sperm per milliliter is considered normal), motility (at least 40% should be swimming), and morphology (at least 4% should be normally shaped). Falling below any of those thresholds reduces the chances of natural conception.
Sperm production is sensitive to a range of influences. Rising body weight is one of the clearest. In men with a BMI over 30, testosterone levels drop significantly compared to men at a healthy weight, and the decline is consistent even after adjusting for age. Varicoceles (enlarged veins in the scrotum), certain medications, heat exposure, and hormonal imbalances can also lower sperm count or quality. Because male-factor issues are so common, a semen analysis should be one of the first tests ordered for any couple having difficulty conceiving, not something reserved for after the female partner has been thoroughly evaluated.
Weight and Lifestyle Factors
Body weight affects fertility on both sides. In women, a BMI above 30 is associated with less regular ovulation, lower implantation rates, and higher miscarriage risk. Being significantly underweight (generally a BMI below 18.5) can suppress ovulation altogether. The relationship isn’t just about extremes: even modest weight changes in either direction can shift hormone levels enough to affect cycle regularity.
Smoking reduces egg quality in women and sperm count in men. Heavy alcohol use disrupts reproductive hormones for both sexes. Chronic stress doesn’t directly “cause” infertility in most cases, but it can delay ovulation and reduce the frequency of sex, both of which lower your monthly odds.
Environmental Chemical Exposures
A growing body of evidence links everyday chemical exposures to reproductive harm. Two of the most studied are phthalates, found in plastics, personal care products, and food packaging, and BPA, common in food containers and can linings. Both are classified as endocrine disruptors, meaning they interfere with your hormonal system.
Phthalates damage developing egg follicles by triggering oxidative stress and promoting cell death, which reduces estrogen production and stalls follicle growth. BPA mimics estrogen and binds directly to estrogen receptors. It suppresses the production of key reproductive hormones in the ovary and can impair the uterine lining’s ability to prepare for implantation. These effects have been demonstrated in both lab and population studies, though the real-world impact depends on the dose and duration of exposure. Reducing contact with soft plastics, choosing glass or stainless steel for food storage, and avoiding heating food in plastic containers are practical steps to lower your exposure.
What a Fertility Workup Looks Like
If you’ve hit the timeline threshold for your age, the standard evaluation covers three areas: ovulation, tubal patency, and sperm quality. For ovulation, blood tests measure hormone levels at specific points in your cycle to confirm whether you’re ovulating and whether your egg reserve is adequate. One of those tests, called AMH, gives an estimate of how many eggs remain in your ovaries, which is especially useful for women in their mid-30s and beyond.
To check whether the fallopian tubes are open, a procedure called a hysterosalpingogram (HSG) is commonly used. A contrast dye is placed into the uterus and tubes, and an X-ray shows whether the dye flows freely through both tubes or gets blocked somewhere along the way. The test also reveals the inner shape of the uterus, which can identify fibroids, polyps, or scarring. For the male partner, a semen analysis covers concentration, motility, and morphology in a single sample.
Together, these tests identify or rule out the most common causes. If everything comes back normal, the diagnosis is unexplained infertility, which accounts for about 30% of infertile couples worldwide. “Unexplained” doesn’t mean nothing is wrong. It means current testing can’t pinpoint the specific barrier, which may involve subtle egg quality issues, sperm-egg interaction problems, or implantation factors that standard tests don’t capture.
Age and Egg Quality
Age is the single strongest predictor of female fertility, and its effects go beyond just the number of eggs remaining. Egg quality, meaning the likelihood that an egg will fertilize normally, develop into a healthy embryo, and implant successfully, declines gradually through the early 30s and more steeply after 35. By 40, the per-cycle chance of conception is roughly half what it was at 30, and the miscarriage rate rises because a higher percentage of embryos carry chromosomal abnormalities.
Male fertility also declines with age, though more gradually. Sperm DNA fragmentation increases over time, which can affect embryo development even when standard semen parameters look normal. Couples where both partners are over 35 face compounding effects from both sides.