Infertility in women has many possible causes, from hormonal imbalances that prevent ovulation to structural problems in the uterus or fallopian tubes. About one in six people of reproductive age worldwide experience infertility, defined as the inability to conceive after 12 months of regular unprotected sex. For most women, the cause falls into one of several well-understood categories, and identifying the right one is the first step toward treatment.
Ovulation Disorders
The most common cause of female infertility is a problem with ovulation. If your ovaries don’t release an egg regularly, or at all, conception can’t happen. Several conditions can disrupt this process.
Polycystic ovary syndrome (PCOS) is the most frequent culprit. It’s an endocrine disorder marked by excess androgens (male-type hormones), irregular menstrual cycles, and sometimes cysts visible on the ovaries via ultrasound. The elevated androgens interfere with the normal hormonal signaling that triggers egg release each month. PCOS can show up as skipped periods, acne, excess facial or body hair, or no obvious symptoms at all. A diagnosis typically requires two of three features: signs of high androgens, irregular cycles, or polycystic ovaries on imaging.
Primary ovarian insufficiency is a less common but more serious condition in which the ovaries stop functioning normally before age 40. It’s sometimes called premature ovarian failure, though the name is somewhat misleading since ovarian function can fluctuate. The hallmark is persistently elevated levels of follicle-stimulating hormone (FSH), the signal your brain sends to prompt egg development. When FSH stays high, it means the ovaries aren’t responding. This can have genetic causes, be triggered by autoimmune conditions, or occur without a clear explanation.
Blocked or Damaged Fallopian Tubes
The fallopian tubes are where sperm meets egg. If they’re blocked or scarred, that meeting can’t happen. Pelvic inflammatory disease (PID), usually caused by sexually transmitted infections like chlamydia or gonorrhea, is the leading cause of tubal damage. The infection travels up from the cervix and inflames the tubes, a condition called salpingitis. Even after the infection clears, scar tissue can permanently narrow or seal the tubes.
Tubal damage can also result from previous abdominal or pelvic surgery, a ruptured appendix, or ectopic pregnancies. In some cases, fluid accumulates in a blocked tube (called a hydrosalpinx), which can further reduce fertility by leaking into the uterus and interfering with implantation.
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 multiple pathways. The lesions trigger chronic inflammation, which creates a hostile environment for eggs, sperm, and embryos. Inflammatory compounds in the pelvic fluid can impair egg maturation, damage the DNA and cellular structures of the egg, and reduce sperm motility.
The effects go beyond the pelvic environment. Women with endometriosis tend to have lower oocyte quality, with eggs that contain fewer mitochondria (the structures that power cell division). Increased oxidative stress inside the ovarian follicles further degrades egg health. The endometrial lining itself may be less receptive to embryo implantation in women with endometriosis, with research showing higher numbers of immature immune cells in the uterine lining compared to fertile women. In moderate to severe cases, endometriotic tissue can also physically distort the reproductive organs, blocking fallopian tubes or encasing the ovaries.
Uterine Factors
Even when ovulation and fertilization occur normally, structural problems inside the uterus can prevent an embryo from implanting. Fibroids, which are noncancerous growths of the uterine muscle, are one of the most common uterine issues. Their impact depends almost entirely on location. Submucosal fibroids, which grow into the uterine cavity, carry the greatest risk. They reduce implantation rates by roughly 70% compared to women without fibroids, likely by distorting the cavity’s shape and altering the lining’s ability to receive an embryo. Intramural fibroids, embedded in the uterine wall, have a more modest effect, reducing implantation by about 38%. Subserosal fibroids, which grow on the outer surface of the uterus, have a negligible impact on fertility.
Uterine polyps, scar tissue from previous surgeries (called Asherman syndrome), and congenital abnormalities in uterine shape can also interfere with implantation or increase the risk of early pregnancy loss.
Age and Egg Quality
Age is one of the strongest predictors of fertility. A woman in her early to mid-20s has a 25 to 30% chance of conceiving in any given month. That probability begins a gradual decline in the early 30s, then drops more steeply after 35. By age 40, the monthly chance of conceiving falls to around 5%.
This decline isn’t just about having fewer eggs. The eggs that remain are more likely to carry chromosomal abnormalities, which increases the chance of failed implantation, miscarriage, or genetic conditions. The decline in egg quality is the primary reason fertility treatments also become less effective with age. This is a biological reality that no lifestyle change can fully counteract, though it varies significantly from person to person.
Hormonal Imbalances Beyond PCOS
Several hormonal conditions outside of PCOS can shut down ovulation. Hyperprolactinemia, a condition where the pituitary gland produces too much prolactin, directly suppresses the hormones needed for ovulation. Prolactin blocks the release of GnRH, the master signal from the brain that triggers FSH and LH, the two hormones that stimulate egg development and release. Without adequate FSH and LH, eggs don’t mature and ovulation doesn’t occur. Women with this condition often experience missed periods or may produce breast milk outside of pregnancy.
Thyroid dysfunction plays a similar disruptive role. An underactive thyroid can cause irregular or absent periods, partly by raising prolactin levels and partly by impairing the hormonal feedback loop that controls the menstrual cycle. Both conditions are treatable with medication, and fertility often returns once hormone levels normalize.
Body Weight and Lifestyle Factors
Body weight has a measurable effect on fertility. Women with a BMI over 30 face roughly three times the risk of infertility compared to women at a normal weight. Excess body fat increases estrogen production, which can disrupt the hormonal balance needed for regular ovulation. It also contributes to insulin resistance, which is closely linked to PCOS.
Being significantly underweight causes problems from the opposite direction. Too little body fat can signal to the brain that conditions aren’t suitable for pregnancy, leading it to suppress reproductive hormones entirely. This is why women with eating disorders or very intense exercise regimens sometimes stop menstruating.
Smoking damages eggs, accelerates the loss of ovarian reserve, and harms the fallopian tubes. Heavy alcohol use and chronic stress can also disrupt ovulation, though their effects are harder to quantify precisely.
When to Seek an Evaluation
The American Society for Reproductive Medicine recommends starting a fertility evaluation after 12 months of trying to conceive if you’re under 35, and after 6 months if you’re 35 or older. The shorter timeline for older women reflects the steeper decline in fertility and the value of identifying treatable causes sooner. If you have known risk factors, such as very irregular periods, a history of pelvic infections, or endometriosis, an earlier evaluation is reasonable regardless of age.
A standard workup typically includes blood tests to check hormone levels, an ultrasound to examine the ovaries and uterus, and an imaging test to check whether the fallopian tubes are open. In many cases, a specific and treatable cause is found. In about 10 to 15% of cases, no clear cause is identified, a diagnosis called unexplained infertility, which still has treatment options available.