The four most common causes of female infertility are ovulation disorders, fallopian tube damage, endometriosis, and uterine abnormalities. Each one interferes with a different step in conception, from releasing an egg to implanting an embryo. Infertility is clinically defined as the failure to achieve pregnancy after 12 months of regular unprotected intercourse, and in the U.S., about 10.4% of women ages 20 to 49 have sought medical help to get pregnant.
1. Ovulation Disorders
For pregnancy to happen, an ovary needs to release a mature egg each cycle. Ovulation disorders disrupt or prevent that release entirely, and they represent the single most common category of female infertility. The most frequent culprit is polycystic ovary syndrome (PCOS), which the World Health Organization identifies as the leading cause of anovulation (absent ovulation) globally.
PCOS is driven by higher-than-normal levels of androgens, sometimes called “male hormones,” though all women produce them in small amounts. When androgen levels climb too high, they throw off the hormonal signals that trigger egg maturation and release. The result is irregular or missing periods, which directly translates to fewer or no chances of conception each year. PCOS can also cause excess facial or body hair and acne, which are often the symptoms that prompt a diagnosis.
A related but distinct condition is primary ovarian insufficiency (POI), where the ovaries lose function before age 40. Women with POI stop producing eggs reliably, and their periods become irregular or stop altogether. It’s diagnosed when follicle-stimulating hormone (FSH) levels are elevated on two separate blood tests taken at least four weeks apart, reflecting that the brain is working harder to stimulate ovaries that aren’t responding. POI affects fertility more severely than PCOS because ovarian reserve, the pool of remaining eggs, is genuinely depleted rather than simply disrupted by hormonal signaling.
2. Fallopian Tube Damage
The fallopian tubes are where sperm meets egg. If one or both tubes are blocked or scarred, that meeting can’t happen, and a fertilized egg may not be able to travel to the uterus for implantation. This is called tubal factor infertility, and it’s most commonly caused by pelvic inflammatory disease (PID).
PID is an infection of the reproductive tract, usually triggered by sexually transmitted infections like chlamydia or gonorrhea. These bacteria cause inflammation in the tubes, and as the body heals, scar tissue forms. The tricky part is that PID can be completely asymptomatic. A woman may never know she had an infection until she has trouble conceiving years later. Among women who do develop PID, 15 to 20% go on to experience infertility, and tubal damage accounts for a large share of those cases. In women ages 18 to 29, a history of PID is associated with a fourfold higher rate of infertility compared to those without one.
Previous abdominal or pelvic surgery can also cause adhesions (bands of scar tissue) that block or kink the tubes. Ectopic pregnancies, where an embryo implants in the tube itself, can damage tubal tissue as well.
3. Endometriosis
Endometriosis occurs when tissue similar to the uterine lining grows outside the uterus, typically on the ovaries, fallopian tubes, and pelvic surfaces. It’s an estrogen-driven, inflammatory disease, and it attacks fertility from multiple directions at once.
The most straightforward damage is physical. Endometriosis triggers adhesions and fibrosis that can warp pelvic anatomy, essentially gluing organs together. Extensive adhesions can block the openings of the fallopian tubes and trap ovaries, preventing egg release and sperm passage. In severe cases, the anatomical damage directly blocks the transport of eggs, sperm, and embryos.
But even when the anatomy looks relatively normal, endometriosis creates a hostile environment for conception. Activated immune cells flood the pelvic and follicular fluid with inflammatory molecules that damage sperm, reduce their motility, and cause DNA damage in both eggs and embryos. This chronic inflammation also degrades egg quality and shrinks ovarian reserve over time, meaning fewer viable eggs are available with each passing cycle.
Perhaps most critically, endometriosis disrupts implantation. The uterine lining needs to undergo specific changes to accept an embryo, a process called decidualization. Endometriosis interferes with this process and alters the immune cell populations in the uterine lining, making the endometrium less receptive. So even if fertilization occurs, the embryo may fail to implant or may be lost very early in pregnancy.
4. Uterine Abnormalities
Once an embryo reaches the uterus, it needs a healthy environment to implant and grow. Structural problems inside the uterus can prevent that from happening. The most common of these are fibroids, which are noncancerous growths in the uterine wall. Fibroids are the sole identifiable cause of infertility in 2 to 3% of women, though they contribute to fertility problems in a larger group.
Location matters enormously. Fibroids that grow on the outer surface of the uterus (subserosal fibroids) don’t appear to affect fertility at all. But fibroids that press into or distort the inner cavity, called submucosal and certain intramural fibroids, are associated with lower pregnancy rates, lower implantation rates, and higher miscarriage rates. They interfere with implantation through several pathways: they alter blood flow to the uterine lining, trigger chronic low-grade inflammation, and reduce key proteins the embryo needs to attach. They also cause abnormal uterine contractions during the critical window when an embryo is trying to implant, potentially dislodging it.
Endometrial polyps, another type of growth inside the uterine cavity, can similarly disrupt implantation. Structural abnormalities present from birth, such as a uterine septum (a wall of tissue dividing the cavity), also fall into this category. These conditions reduce the usable space inside the uterus and can interfere with both implantation and the ability to carry a pregnancy to term.
How These Causes Overlap
These four categories aren’t always neatly separate. A woman with endometriosis may also have tubal damage from adhesions and compromised egg quality from ovarian inflammation, meaning two or three of these factors are working against her simultaneously. PCOS can coexist with uterine fibroids. And in many cases, more than one contributing factor is identified during a fertility workup, which is why evaluation typically looks at ovulation, tubal patency, and uterine structure together rather than testing one cause at a time.
The recommended timeline for evaluation depends on age. Women under 35 are generally advised to seek evaluation after 12 months of trying to conceive. Women over 35 should consider evaluation after 6 months, and women 40 and older benefit from more immediate assessment, since both egg quantity and quality decline with age regardless of other factors.