The 6 Main Infertility Causes: A 2026 Evidence-Based Guide for Couples

Infertility causes range from hormonal and structural problems to genetic factors, lifestyle choices, age, and conditions that remain unexplained even after a full workup. According to the CDC’s most recent National Survey of Family Growth, 13.4% of US women ages 15–49 have impaired fecundity, and 8.5% of married women are clinically infertile. About 11.4% of US men ages 15–49 also report some form of infertility.

Infertility is medically defined as the inability to conceive after 12 months of unprotected sex (or 6 months if the woman is 35 or older). Knowing what’s behind it is the first step toward the right treatment — and toward avoiding wasted time, money, and stress.

Infertility causes by gender: who’s affected and why

The classic “one third / one third / one third” rule still holds in 2026, confirmed by the CDC FastStats on Infertility and the Eunice Kennedy Shriver National Institute of Child Health and Human Development:

• One third of cases: female factor.
• One third of cases: male factor.
• One third of cases: combined factors or unexplained infertility.

This is why the AUA/ASRM 2024 amendment of its male infertility guidelines is explicit: both partners must be evaluated in parallel, never sequentially. Diagnosing the woman first while assuming the man is fine is the most common diagnostic error in primary care.

Primary vs. secondary infertility

Two clinical categories help frame the conversation with your reproductive endocrinologist:

• Primary infertility: the inability to conceive a first child after 12 months (or 6 months if the woman is 35+). About 19.4% of married US women with no prior births are infertile, per the CDC.
• Secondary infertility: trouble conceiving a subsequent child after a previous pregnancy. About 6% of married women with at least one birth meet this definition. Often surprising and emotionally difficult, it is just as legitimate as primary infertility.

The 6 main infertility causes you need to know

Per the 2025 ASRM, ACOG and NIH evidence-based fact sheet, infertility causes cluster into six broad categories. Understanding them is essential before any treatment decision.

1. Ovulatory disorders

Ovulation problems explain about 40% of female infertility cases. The most common is Polycystic Ovary Syndrome (PCOS), which affects 1 in 10 women of reproductive age. Other ovulatory infertility causes include hypothalamic dysfunction, premature ovarian insufficiency, hyperprolactinemia, and thyroid disorders. Most respond well to ovulation induction with letrozole or clomiphene.

2. Tubal and structural factors

Blocked or damaged fallopian tubes account for roughly 20% of female infertility. The leading cause is pelvic inflammatory disease (PID), often triggered by untreated chlamydia or gonorrhea. Other structural infertility causes include uterine fibroids, polyps, intrauterine adhesions (Asherman’s syndrome), and congenital abnormalities like a uterine septum. Hysterosalpingography and saline sonography are the standard diagnostic tools.

3. Endometriosis

Endometriosis — when uterine-like tissue grows outside the uterus — affects about 10% of women of reproductive age and is one of the most under-diagnosed infertility causes. It can take 7 to 10 years to receive a confirmed diagnosis. Symptoms include severe menstrual pain, painful intercourse, and chronic pelvic pain, but up to 25% of women with endometriosis have no symptoms at all and only learn about it during a fertility workup.

4. Male factor infertility

Male infertility causes include low sperm count (oligospermia), poor sperm motility (asthenospermia), abnormal morphology (teratospermia), varicocele (the most common reversible cause), genetic conditions like Klinefelter syndrome, undescended testicles, ejaculatory disorders, and prior chemotherapy or radiation. A semen analysis is the first test — inexpensive, fast, and high-yield. Couples with a male partner aged 40 or over are also more likely to have difficulty conceiving, per the CDC, and the wider trend of global decline in male fertility over the past 50 years adds urgency to early evaluation.

5. Age-related infertility causes

Female fertility starts declining gradually at age 30, more sharply after 35, and dramatically after 40. A 25-year-old woman has an 18% chance of conceiving per cycle; by 40 it drops to 7%. Egg quality and quantity (ovarian reserve) both decline with age, and the rate of chromosomally abnormal embryos rises from about 31% under 35 to over 70% after 40. Sperm quality also declines after 40, with measurable drops in motility and DNA integrity.

6. Unexplained infertility

About 15% to 30% of couples complete a full workup with no identifiable diagnosis. This is called unexplained infertility. It does not mean nothing is wrong — it means current diagnostic tools cannot detect the underlying issue. Treatment usually moves directly to ovulation induction with intrauterine insemination (IUI), or to IVF after 3 to 6 failed IUI cycles. See our guide on IVF success rate for realistic age-stratified outcomes.

Lifestyle and environmental infertility causes

Modifiable factors can affect both partners. Per ASRM and the Office on Women’s Health, the strongest evidence-backed lifestyle infertility causes include:

• Smoking: reduces female fertility by about 13% per cigarette/day and lowers sperm count and motility.
• Excess body weight (BMI ≥ 30 or ≤ 18.5): disrupts ovulation and reduces sperm quality. Even a 5–10% weight loss can restore ovulation in many women with PCOS.
• Alcohol: heavy drinking (>14 drinks/week) is linked to ovulatory dysfunction and reduced sperm parameters.
• Chronic stress: elevated cortisol disrupts the hypothalamic-pituitary-ovarian axis. Stress alone rarely causes infertility but can worsen other factors.
• Environmental toxins: pesticides, BPA, phthalates, and heavy metals are increasingly suspected contributors.
• Heat exposure for men: hot tubs, saunas, and laptops on the lap reduce sperm production.

Timing of intercourse: a frequently overlooked factor

Even with both partners perfectly healthy, the chance of conceiving in any given month is only about 25%. The fertile window is just 5–6 days per cycle: 5 days before ovulation through 24 hours after. Sperm survive up to 72 hours in the female reproductive tract; the egg lives only 12–24 hours after ovulation.

The simplest evidence-based strategies: have intercourse every 1–2 days throughout the cycle, or use an ovulation monitor to pinpoint your fertile window.

When to see a fertility specialist

The ASRM and ACOG agree on these clear thresholds:

• If the woman is under 35: after 12 months of unprotected sex without conception.
• If the woman is 35 to 39: after 6 months.
• If the woman is 40 or older: immediately, without a waiting period.
• Earlier if there are known risk factors: irregular periods, history of PID, prior cancer treatment, two or more miscarriages, or known male factor issues.

Frequently asked questions about infertility causes

What are the most common infertility causes in women?

Ovulation disorders (about 40%), tubal and structural factors (~20%), endometriosis (~10%), and age-related egg quality decline. PCOS alone accounts for 70–80% of anovulatory infertility cases.

What are the most common infertility causes in men?

Low sperm count, low motility, and abnormal morphology together account for the majority. Varicocele is the most common reversible cause and is found in about 40% of infertile men. Genetic conditions, hormonal imbalances, and previous infections also contribute.

Can infertility causes be reversed?

Many can. Ovulation disorders respond well to medication. Varicocele can be surgically corrected. Lifestyle changes (weight, smoking, alcohol) often restore fertility within 6–12 months. Tubal blockages may require surgery or IVF. Genetic and severe age-related cases usually require assisted reproduction such as IVF, donor eggs, or donor sperm.

Is unexplained infertility really “unexplained”?

It means standard tests found no cause. Roughly 15–30% of couples receive this diagnosis. Treatment typically starts with 3 IUI cycles using ovulation stimulation, then IVF if unsuccessful. Many “unexplained” cases are likely due to subtle egg or sperm quality issues current tests can’t yet detect.

Does stress alone cause infertility?

Stress alone rarely causes infertility, but it can worsen other underlying infertility causes by disrupting the hypothalamic-pituitary-ovarian axis. Stress-management techniques (CBT, mindfulness, exercise) are recommended adjuncts during fertility treatment but are not standalone solutions.

Whether you’re starting your fertility journey, considering donor sperm, or exploring co-parenting as an alternative path to parenthood, you don’t have to figure it out alone. Join CoParents.com today and connect with more than 150,000 members worldwide who are building their families through co-parenting, sperm donation, and shared parenthood.