A Complete Guide to the Causes of Infertility and How to Overcome Them

The most common causes of infertility include hormonal imbalances such as PCOS, structural problems in the reproductive organs, declining egg or sperm quality with age, lifestyle factors like smoking and obesity, and underlying medical conditions such as endometriosis or diabetes. In the UK, around 1 in 7 couples experience difficulty conceiving, and in approximately 25% of cases no identifiable cause is found. Understanding what may be affecting your fertility is the first step toward finding the right treatment and building your family.

What Are the Most Common Causes of Infertility in Women?

Female infertility accounts for roughly one third of all infertility cases. The causes of infertility in women are varied, but ovulation disorders are by far the most frequent. Polycystic ovary syndrome (PCOS) is the leading cause, affecting approximately 1 in 10 women of reproductive age in the UK. PCOS disrupts the normal hormonal cycle and prevents the ovaries from releasing eggs regularly, making conception unpredictable.

Other hormonal causes of infertility include thyroid disorders (both overactive and underactive thyroid can interfere with ovulation), hyperprolactinaemia (excess prolactin production), and premature ovarian insufficiency, where the ovaries stop functioning before the age of 40. Hormonal imbalances can often be identified through blood tests and treated with medication. If you suspect a hormonal issue, speak to your GP — early diagnosis significantly improves outcomes.

Structural problems are another major category. Blocked or damaged fallopian tubes, often caused by pelvic inflammatory disease or previous surgery, prevent sperm from reaching the egg. Endometriosis — where tissue similar to the uterine lining grows outside the womb — affects approximately 1 in 10 women and is a well-documented cause of infertility. Uterine fibroids, polyps and congenital abnormalities of the uterus can also interfere with embryo implantation. Some structural issues can be addressed through surgery, while others may require IVF treatment to bypass the problem entirely.

What Are the Causes of Infertility in Men?

Male factor infertility contributes to around 30 to 50% of all infertility cases. The most common causes of infertility in men relate to sperm production and quality. A low sperm count (oligozoospermia), defined as fewer than 15 million sperm per millilitre of semen, reduces the chances of an egg being fertilised. Poor sperm motility (asthenozoospermia) means sperm cannot swim effectively toward the egg, while abnormal sperm shape (teratozoospermia) makes it harder for sperm to penetrate the egg.

These issues can be caused by varicoceles (swollen veins in the scrotum, found in approximately 15% of men), hormonal imbalances, genetic conditions, infections, or lifestyle factors. Blockages in the vas deferens or epididymis can prevent sperm from being ejaculated, though surgical correction is sometimes possible. Undescended testicles, a history of testicular injury and certain medications (including some antidepressants and testosterone supplements) can also impair sperm production.

If you have been trying to conceive for over a year without success, a semen analysis is one of the first tests your GP will recommend. This simple laboratory test assesses sperm count, motility, morphology and volume, and can identify most causes of infertility related to sperm. For more detail, read our guide on signs of male infertility.

How Does Age Affect Fertility?

Age is one of the most significant and unavoidable causes of infertility for both women and men. Among all causes of infertility, age-related decline is the hardest to address because it affects both egg quantity and quality simultaneously. According to the British Fertility Society, a woman’s chance of conceiving naturally falls from approximately 20% per cycle at age 30 to around 5% per cycle by age 40. This decline is driven by both a reduction in the number of remaining eggs and a decline in egg quality, which also increases the risk of miscarriage and chromosomal abnormalities.

Male fertility also declines with age, though more gradually. Sperm quality, including motility and DNA integrity, tends to decrease after age 40. Older fathers also face a slightly higher risk of passing on genetic mutations. While age-related fertility decline cannot be reversed, assisted reproductive technologies such as IVF can improve the odds. Women who know they want children later may also consider egg freezing as part of their fertility planning.

How Do Lifestyle Factors Cause Infertility?

Lifestyle choices have a direct and measurable impact on reproductive health. According to the NHS, the following lifestyle factors are recognised causes of infertility:

• Smoking damages both eggs and sperm, reduces IVF success rates and increases the risk of miscarriage. Passive smoking is also harmful. Quitting tobacco is one of the most effective steps you can take to improve your fertility.
• Weight plays a critical role. Being overweight or obese (BMI over 30) disrupts hormone balance and can prevent ovulation in women. Being severely underweight can also stop periods entirely. In men, obesity is associated with lower sperm count and reduced testosterone.
• Alcohol consumption reduces fertility in both sexes. The NHS recommends avoiding alcohol entirely when trying to conceive, as even moderate drinking can affect sperm quality and ovulation.
• Stress can interfere with ovulation and sperm production, and may reduce sex drive, making conception less likely. Relaxation techniques, exercise and counselling can all help manage stress during the fertility journey.

The encouraging news is that lifestyle-related causes of infertility are often reversible. Maintaining a healthy BMI, quitting smoking, reducing alcohol intake and taking recommended supplements (folic acid for women, zinc and selenium for men) can all meaningfully improve your chances of conceiving.

What Medical Conditions Can Cause Infertility?

Several underlying medical conditions rank among the less obvious but significant causes of infertility, sometimes without presenting any symptoms. Endometriosis, which affects approximately 10% of women of reproductive age, can cause scarring and adhesions that block the fallopian tubes or impair egg quality. Pelvic inflammatory disease (PID), usually caused by untreated sexually transmitted infections such as chlamydia, can permanently damage the reproductive tract if not treated early.

Diabetes (both type 1 and type 2) can affect fertility in men and women by disrupting hormone levels and causing oxidative stress in reproductive cells. Autoimmune conditions, coeliac disease and thyroid disorders are also linked to reduced fertility. Certain medications — including some treatments for epilepsy, high blood pressure, depression and cancer — can impair ovulation or sperm production. If you are taking any medication and planning a pregnancy, speak to your doctor before trying to conceive. Many alternatives exist that are safer for fertility.

In about 25% of infertility cases, no specific cause can be identified despite thorough testing. This is known as unexplained infertility. It does not mean nothing can be done — many couples with unexplained infertility go on to conceive successfully through treatments such as IUI or IVF.

What Treatment Options Are Available for the Causes of Infertility?

Treatment depends entirely on the diagnosed cause. For ovulation disorders, medications such as clomifene citrate or letrozole are often the first line of treatment, stimulating the ovaries to release eggs. Hormonal therapies can address thyroid issues, hyperprolactinaemia and other endocrine imbalances. Surgery may be recommended for blocked fallopian tubes, endometriosis, fibroids or varicoceles.

When simpler treatments are unsuccessful, assisted reproductive technologies offer additional options. Intrauterine insemination (IUI) places prepared sperm directly into the uterus and is effective for mild male factor infertility, unexplained infertility and couples using donor sperm. In vitro fertilisation (IVF) offers the highest per-cycle success rates and is recommended after failed IUI cycles, for severe male factor infertility, or when donated eggs are needed. NICE guidelines recommend that women under 43 should be offered up to 3 cycles of IVF on the NHS, though actual access varies by region.

For men with very low sperm counts, intracytoplasmic sperm injection (ICSI) — where a single sperm is injected directly into an egg — can achieve fertilisation even when natural conception would be impossible. Couples and individuals who cannot conceive with their own gametes may also consider embryo donation, egg donation, or using a sperm donor through a platform like CoParents.co.uk, a co-parenting and sperm donation network with over 150,000 users since 2008.

Where Can You Find Support for Infertility?

Dealing with infertility is emotionally challenging, and seeking support is an important part of the journey. Your GP is the best starting point — they can run initial tests, refer you to a fertility specialist and advise on NHS-funded treatment options in your area. The HFEA website provides information on licensed clinics, treatment success rates and patient rights.

Charities such as Fertility Network UK offer free support groups, helplines and peer-to-peer communities where you can share your experiences. Many fertility clinics include counselling as part of their treatment packages. If the causes of infertility you face cannot be resolved through medical treatment, alternative paths to parenthood — including co-parenting, donor conception and adoption — are more accessible than ever.

Frequently Asked Questions About Causes of Infertility

What is the most common cause of infertility in the UK?

Ovulation disorders, particularly PCOS, are the most common causes of infertility in women. In men, low sperm count and poor sperm motility are the leading factors. Overall, male and female factors each account for roughly a third of cases, with the remaining third being unexplained or due to combined factors.

Can stress really cause infertility?

Severe or chronic stress can disrupt ovulation and reduce sperm production by affecting hormone levels. While stress alone is unlikely to be the sole cause of infertility, managing it through exercise, mindfulness or counselling can improve your overall chances of conceiving.

When should I see a doctor about infertility?

The NHS recommends seeing your GP if you have not conceived after 12 months of regular unprotected sex. Women over 36 and anyone with known risk factors (irregular periods, previous pelvic infections, known endometriosis) should seek advice after 6 months. Men experiencing ejaculation problems, testicular pain or a history of undescended testicles should also consult their doctor early.

Can causes of infertility be prevented?

Some causes of infertility can be reduced through lifestyle choices: maintaining a healthy weight, avoiding smoking and excessive alcohol, practising safe sex to prevent STIs, and getting regular health check-ups. However, genetic factors, age-related decline and some medical conditions cannot be prevented — only managed through appropriate treatment.

If you are exploring alternative paths to parenthood while navigating fertility challenges, create your free profile on CoParents.co.uk to connect with sperm donors and co-parents across the UK and start your journey toward becoming a parent.