Infertility is defined as the inability to conceive after twelve months of regular, unprotected intercourse, or six months when the woman is above 35. It affects a significant proportion of couples trying to conceive, and yet it remains a condition many people approach with assumptions that do not reflect the clinical reality. The most persistent of these assumptions is that infertility is primarily a female problem. It is not.

The male factor substantially contributes to about 50% of all cases of infertility. The male is solely responsible in about 20% of cases and is a contributing factor in another 30 to 40% of all cases. As male and female causes often coexist, it is essential that both partners are investigated and managed together.

Understanding the most common causes on both sides is the starting point for anyone navigating a fertility evaluation.

Female infertility causes

Ovulation disorders

Most cases of female infertility are caused by problems with ovulation. Without ovulation, there are no eggs to be fertilised. PCOS is a hormone imbalance problem which can interfere with normal ovulation and is the most common female infertility causes.

The most common overall cause of female infertility is failure to ovulate, which occurs in 40% of women with infertility. Polycystic ovary syndrome produces irregular or absent ovulation through insulin resistance and elevated androgen levels. Primary ovarian insufficiency, where the ovaries stop functioning before the age of 40, is another cause of ovulatory failure and carries additional implications for hormonal health and bone density.

Tubal disease

Blocked or damaged fallopian tubes prevent sperm from reaching the egg or prevent a fertilised egg from travelling to the uterus. The most common causes of tubal damage include previous pelvic inflammatory disease, often from chlamydial infection, previous ectopic pregnancy, and endometriosis affecting the tubes. Sexually transmitted infections such as chlamydia can damage the fallopian tubes, and some other STIs may also cause infertility.

Endometriosis

Endometriosis is a disease in which tissue identical to the lining of the uterus grows outside the uterus, on the ovaries, tubes and other pelvic structures. This causes inflammation, adhesions, and sometimes direct injury to the tubes and ovaries. Specifically, even moderate endometriosis can disrupt fertility through inflammatory mechanisms, which can damage the quality of the egg and the implantation of the embryo. Endometriosis is one of the more difficult causes of female infertility to diagnose and treat. 

Uterine factors

Structural problems such as the presence of abnormal tissue in the fallopian tubes or uterus are another common reason for female infertility. Structural issues prevent the egg from meeting the sperm or prevent the embryo from implanting.

Fibroids, polyps, a septate uterus, or intrauterine adhesions from previous procedures or infections can all interfere with implantation and early pregnancy maintenance. These are usually identified during a uterine assessment as part of the fertility workup.

Diminished ovarian reserve

A woman's ovarian reserve, representing the quantity and quality of eggs remaining, declines with age and can be reduced prematurely by genetics, previous ovarian surgery, or autoimmune conditions. Low ovarian reserve reduces the number of eggs available for stimulation and retrieval, and the eggs that remain are more likely to carry chromosomal abnormalities.

Male infertility causes

Sperm abnormalities

The most common findings in male infertility relate directly to the sperm itself. These include:

• Low sperm count, where the number of sperm in the ejaculate falls below the threshold needed for reliable fertilisation
• Poor sperm motility, where sperm cannot swim effectively enough to reach and penetrate the egg
• Abnormal sperm morphology, where the shape of the sperm impairs its ability to fertilise
• Azoospermia, the complete absence of sperm in the ejaculate, which occurs in around 10 to 15% of infertile men and may reflect either a production problem or an obstruction in the reproductive tract

Varicocele

Varicocele is a swelling of the veins that drain the testicle. It is the most common reversible cause of male infertility and may lead to reduced sperm quantity and quality. The enlarged veins raise scrotal temperature, impairing sperm production. Surgical repair of a varicocele can improve semen parameters in a meaningful proportion of men.

Hormonal imbalances

The hormonal axis regulating sperm production involves signals from the brain and pituitary gland to the testicles. Disruption at any level, whether from a pituitary tumour, thyroid dysfunction, or conditions affecting testosterone production, can reduce sperm output significantly. Exogenous testosterone, including anabolic steroids used for body building, suppresses this axis and can cause severe reductions in sperm count.

Genetic causes

Klinefelter syndrome, Y chromosome microdeletions, and cystic fibrosis gene mutations are among the genetic causes of male infertility. Klinefelter syndrome occurs when a boy is born with an extra copy of the X chromosome and often is not diagnosed until adulthood. Men with congenital bilateral absence of the vas deferens, linked to cystic fibrosis mutations, produce sperm in the testicles but cannot deliver it through ejaculation.

Unexplained infertility

Even after a thorough investigation of both partners, a proportion of couples are given a diagnosis of unexplained infertility, where no specific cause can be identified with current diagnostic tools.

One third of infertility cases in heterosexual couples are due to male infertility, one third to female infertility, and one third to both partners or entirely unknown causes. Unexplained infertility does not mean nothing is wrong. It means the cause has not yet been identified. These couples are typically offered assisted reproductive treatment with the expectation that many will achieve pregnancy through IUI or IVF.

Takeaways

The causes of infertility are distributed roughly equally between female infertility causes, male infertility causes, and unexplained factors. Female causes include ovulation disorders, tubal disease, endometriosis, uterine abnormalities, and diminished ovarian reserve. Male causes include sperm abnormalities, varicocele, hormonal imbalances, and genetic conditions. 

Both partners should be investigated simultaneously from the outset, as causes frequently coexist. Unexplained infertility affects a significant proportion of couples and does not preclude successful treatment. Anyone who has been trying to conceive for twelve months without success, or six months above the age of 35, should seek a fertility evaluation without further delay.