Infertility has many possible causes, and the cause may originate in the person producing eggs, the person producing sperm, both partners, or remain unexplained. Major categories include ovulatory disorders, problems with sperm production or function, blocked or damaged fallopian tubes, disorders affecting the uterus or endometrium, endometriosis, hormonal disorders, sexual or ejaculation difficulties, previous reproductive infections, certain treatments and age-related changes. The NHS identifies lack of regular ovulation, poor-quality semen, blocked or damaged fallopian tubes and endometriosis among common causes. In approximately one quarter of cases, no specific cause can be identified despite investigation.
Ovulatory disorders are a major cause of infertility because pregnancy requires an egg to be released. Polycystic ovary syndrome can disrupt the hormonal processes controlling ovulation, while thyroid disorders can also interfere with reproductive hormone regulation. Premature ovarian insufficiency can cause ovarian function to decline before age 40. Some people have absent ovulation, while others ovulate irregularly, producing fewer opportunities for conception. Ovulatory problems may be suggested by irregular or absent periods, although regular bleeding does not absolutely prove that ovulation occurs normally.
Tubal and pelvic disorders can prevent fertilisation or interfere with the movement of an egg or embryo. Pelvic inflammatory disease can damage and scar the fallopian tubes, particularly after infections such as chlamydia or gonorrhoea. Endometriosis involves tissue resembling the endometrium growing outside the uterine cavity and can affect the ovaries, tubes and surrounding pelvic structures. Previous pelvic surgery can cause adhesions or scarring. Fibroids are benign uterine growths that can sometimes interfere with implantation or obstruct a fallopian tube.
Male-factor infertility can result from abnormalities of semen or sperm. A semen sample may contain too few sperm, sperm may have impaired motility, or a large proportion may have abnormal morphology. Testicular damage can result from infection, cancer, surgery, trauma or congenital abnormalities such as undescended testes. Hormonal disorders involving testosterone and the reproductive endocrine system can reduce sperm production. Ejaculation disorders can prevent sperm from being deposited appropriately during intercourse. Anabolic steroid misuse is another important cause because external androgens can suppress the hormonal signals required for normal sperm production.
Medications and medical treatments can sometimes reduce fertility. Chemotherapy can damage ovarian or testicular function, while certain medicines can temporarily affect sperm production or ovulation. Some long-term or high-dose non-steroidal anti-inflammatory drug use can interfere with ovulation, and certain antipsychotic medicines can alter reproductive hormone function. Sulfasalazine can reduce sperm count temporarily, while anabolic steroids can substantially suppress sperm production. The effect depends on the particular medicine, dose, duration and individual susceptibility, so prescribed medication should not be stopped without medical advice.
Lifestyle and environmental factors can contribute to infertility without being the sole cause. Smoking can impair fertility in both sexes, excessive alcohol can adversely affect reproductive health, and recreational drugs such as cannabis and cocaine can interfere with reproductive function. Obesity can reduce fertility, while severe underweight can disrupt ovulation. Exposure to certain pesticides, solvents and metals may affect reproductive function. These factors can operate through hormonal disruption, oxidative stress, impaired gamete production, altered sexual function or damage to reproductive tissues. Their effects vary substantially between individuals.
Age-related reproductive change is another important cause or contributor. In people with ovaries, fertility declines because ovarian reserve decreases and the remaining eggs are more likely to have chromosomal abnormalities as age increases. The decline accelerates during the mid-30s and becomes more pronounced later. Age can also affect sperm quality, although the pattern differs from ovarian ageing. Age therefore interacts with other causes rather than operating as an isolated diagnosis. A person may have a normal reproductive examination yet have reduced probability of conception because the biological efficiency of reproduction changes with age.
Some infertility remains unexplained because reproductive processes are biologically complex and available tests cannot measure every relevant step. Standard investigations can assess ovulation, sperm characteristics, tubal patency and major anatomical abnormalities, but they cannot perfectly evaluate every aspect of egg quality, sperm function, fertilisation competence, embryo development or implantation. Consequently, a normal test result does not mean that every component of fertility is normal. Causes can also coexist, such as mild ovulatory dysfunction combined with reduced sperm quality. Treatment is therefore guided by the overall clinical picture rather than by a single abnormal result.
