Infertility in women has many possible causes, ranging from hormonal imbalances that prevent ovulation to physical blockages in the reproductive tract. It’s defined as being unable to get pregnant after at least one year of trying, or six months if you’re over 35. Recurrent miscarriages also fall under the infertility umbrella. In most cases, a specific cause can be identified and treated, but sometimes multiple factors overlap.
Ovulation Problems
The most common cause of female infertility is a failure to ovulate regularly. Without the release of an egg each cycle, conception simply can’t happen. Polycystic ovary syndrome (PCOS) is the leading culprit here. In PCOS, the body produces higher than normal levels of androgens (sometimes called “male hormones,” though women naturally produce them too). These elevated androgens disrupt the hormonal signaling that triggers ovulation, leading to irregular or absent periods.
PCOS isn’t the only reason ovulation can stall. High levels of prolactin, a hormone normally involved in milk production, can interfere with the production of estrogen and progesterone, which changes or stops ovulation entirely. An underactive thyroid is one of the most common triggers for elevated prolactin. Treating the thyroid problem with replacement hormone typically brings prolactin levels back to normal and restores ovulation on its own.
Premature Ovarian Insufficiency
Some women’s ovaries slow down or stop functioning well before the typical age of menopause. This condition, called primary ovarian insufficiency (POI), means the ovaries no longer release eggs reliably or produce adequate hormones. In about 90% of cases, no clear cause is found. Known triggers include genetic conditions like Turner syndrome and Fragile X syndrome, autoimmune diseases (where the immune system mistakenly attacks ovarian tissue), and prior chemotherapy or radiation therapy. Exposure to certain toxins, including cigarette smoke, chemicals, and pesticides, has also been linked to POI.
POI is typically identified through blood tests measuring hormone levels, sometimes combined with genetic testing and a pelvic ultrasound. It can affect women in their 20s and 30s, long before they’d expect any decline in fertility.
Blocked or Damaged Fallopian Tubes
The fallopian tubes are where an egg meets sperm, so any blockage or scarring there can prevent conception entirely. Pelvic inflammatory disease (PID) is the most frequent cause of this damage. PID is an infection of the reproductive organs, most often triggered by sexually transmitted infections like chlamydia and gonorrhea, though non-sexually transmitted infections can cause it too. The infection creates scar tissue both inside and outside the fallopian tubes, which can partially or completely block them.
What makes PID particularly dangerous for fertility is that it can be mild enough to go unnoticed. Many women don’t realize they’ve had it until they struggle to conceive and imaging reveals tubal scarring. Previous abdominal or pelvic surgery, a ruptured appendix, or a history of ectopic pregnancy can also leave scar tissue that affects the tubes.
Endometriosis
Endometriosis occurs when tissue similar to the uterine lining grows outside the uterus, often on the ovaries, fallopian tubes, or the tissue lining the pelvis. It affects fertility through several pathways at once. The condition creates chronic inflammation in the pelvic environment, which damages egg quality and disrupts the normal development of follicles (the tiny sacs in the ovaries where eggs mature). It also alters immune function and hormonal balance in the surrounding fluid, impairing fertilization even when an egg and sperm do meet.
Beyond these chemical effects, endometriosis can physically distort the anatomy of the reproductive organs, pulling the fallopian tubes out of position or creating adhesions that block the egg’s path. The condition also appears to reduce the uterine lining’s ability to accept an embryo, making implantation less likely even if fertilization occurs. This combination of poor egg quality, impaired fertilization, and reduced implantation success is why endometriosis is one of the more challenging fertility obstacles to overcome.
Uterine and Cervical Problems
Growths inside the uterus can interfere with pregnancy even when ovulation and the fallopian tubes are perfectly normal. Uterine fibroids, particularly those that push into the uterine cavity (called submucosal fibroids), can distort the space where an embryo needs to implant. Endometrial polyps, small overgrowths of the uterine lining, create localized chronic inflammation that reduces the lining’s receptivity to an embryo. Research shows that polyps decrease implantation rates, increase miscarriage risk, and can even interfere with sperm transport through the uterus.
The inflammation from polyps involves elevated levels of inflammatory compounds and immune cells called mast cells, which release signals that make the uterine environment less hospitable. Structural abnormalities of the uterus present from birth, such as a uterus divided by a wall of tissue (septate uterus), can also reduce the chances of successful implantation or carrying a pregnancy to term. Cervical problems are less common but can contribute: scarring from previous procedures or abnormalities in cervical mucus can create a barrier that prevents sperm from reaching the egg.
Body Weight and Lifestyle Factors
Weight has a measurable effect on fertility. A higher BMI is associated with longer time to conception, a greater likelihood of needing fertility treatment, and higher rates of miscarriage. Excess body fat alters hormone levels, particularly estrogen, which can disrupt ovulation. Being significantly underweight carries similar risks, as too little body fat can cause the brain to reduce or shut down reproductive hormone production altogether.
Smoking also extends the time it takes to conceive and accelerates the loss of eggs from the ovaries, effectively aging the reproductive system faster than the calendar would suggest. The chemicals in cigarette smoke are directly toxic to eggs. Excessive exercise, chronic stress, and certain environmental exposures (pesticides, industrial chemicals) round out the lifestyle factors that can quietly erode fertility over time.
Age and Egg Quality
Age is the single most significant factor in female fertility, and it’s the one most often underestimated. Women are born with all the eggs they’ll ever have, roughly one to two million at birth, declining to about 300,000 by puberty. Both the number and the genetic quality of those eggs decrease steadily with time. The decline accelerates noticeably after age 35 and becomes steep after 40. Older eggs are more likely to have chromosomal abnormalities, which increases the risk of failed implantation, miscarriage, and genetic conditions.
This isn’t just about running out of eggs. The eggs that remain become less efficient at the cellular processes required for healthy fertilization and early embryo development. A 25-year-old has roughly a 25 to 30 percent chance of conceiving in any given cycle. By 40, that drops to around 5 percent per cycle.
Primary vs. Secondary Infertility
Infertility doesn’t only affect women who have never been pregnant. Secondary infertility, the inability to conceive or carry a pregnancy after previously having a child, is actually more common. A large global analysis published in The Lancet found that about 10.5% of women who had previously given birth were unable to have another child, compared to 1.9% of women experiencing primary infertility (never having achieved a live birth). The causes of secondary infertility are largely the same: age-related decline, new hormonal imbalances, damage from a previous delivery or infection, or conditions like endometriosis that have worsened over time. If you conceived easily before, it doesn’t guarantee the same experience next time.
Unexplained Infertility
In roughly 15 to 30 percent of couples who undergo a full evaluation, no clear cause is found. This doesn’t mean nothing is wrong. It means current testing isn’t sensitive enough to detect the problem. Subtle issues with egg quality, sperm-egg interaction, embryo development, or the uterine lining’s receptivity may be at play but fall below the threshold of standard diagnostic tools. Unexplained infertility can be one of the more frustrating diagnoses, but many women in this category still conceive with treatment or, in some cases, on their own over time.