Infertility can be caused by many different things. For 25% of couples, a cause can't be identified.
This page covers the possible causes of infertility in men and women.
Infertility in women
Infertility is most commonly caused by problems with ovulation, the monthly release of an egg.
Some problems stop an egg being released at all, while others prevent an egg being released during some cycles but not others.
Ovulation problems can be a result of:
- polycystic ovary syndrome (PCOS)
- thyroid problems – both an overactive thyroid gland and an underactive thyroid gland can prevent ovulation
- premature ovarian failure – where a woman's ovaries stop working before the age of 40
Scarring from surgery
Pelvic surgery can damage and scar the fallopian tubes, which link the ovaries to the womb.
Cervical surgery can also sometimes cause scarring or shorten the neck of the womb (the cervix).
Cervical mucus problems
When you're ovulating, mucus in your cervix becomes thinner so sperm can swim through it more easily. If there's a problem with the mucus, it can make it harder to conceive.
Non-cancerous growths called fibroids in or around the womb can affect fertility. In some cases, they may prevent a fertilised egg attaching itself to the womb, or they may block a fallopian tube.
Endometriosis is a condition where small pieces of the womb lining (the endometrium) start growing in other places, such as the ovaries.
This can damage the ovaries or fallopian tubes and cause fertility problems.
Pelvic inflammatory disease
Pelvic inflammatory disease (PID) is an infection of the upper female genital tract, which includes the womb, fallopian tubes and ovaries.
It's often caused by a sexually transmitted infection (STI). PID can damage and scar the fallopian tubes, making it virtually impossible for an egg to travel down into the womb.
Some women choose to be sterilised if they don't want to have any more children.
Sterilisation involves blocking the fallopian tubes to make it impossible for an egg to travel to the womb.
It's rarely reversible – if you do have a sterilisation reversed, you won't necessarily become fertile again.
Medicines and drugs
The side effects of some types of medicines and drugs can affect your fertility.
- non-steroidal anti-inflammatory drugs (NSAIDs) – long-term use or a high dosage of NSAIDs, such as ibuprofen or aspirin, can make it more difficult to conceive
- chemotherapy – medicines used for chemotherapy can sometimes cause ovarian failure, which means your ovaries will no longer be able to function properly
- neuroleptic medicines – antipsychotic medicines often used to treat psychosis; they can sometimes cause missed periods or infertility
- spironolactone – a type of medicine used to treat fluid retention (oedema); fertility should recover around 2 months after you stop taking spironolactone
Illegal drugs, such as marijuana and cocaine, can seriously affect fertility and make ovulation more difficult.
Infertility in men
Semen and sperm
The most common cause of infertility in men is poor-quality semen, the fluid containing sperm that's ejaculated during sex.
Possible reasons for abnormal semen include:
- a lack of sperm – you may have a very low sperm count or no sperm at all
- sperm that aren't moving properly – this will make it harder for sperm to swim to the egg
- abnormal sperm – sperm can sometimes be an abnormal shape, making it harder for them to move and fertilise an egg
Many cases of abnormal semen are unexplained.
There's a link between increased temperature of the scrotum and reduced semen quality, but it's uncertain whether wearing loose-fitting underwear improves fertility.
The testicles produce and store sperm. If they're damaged, it can seriously affect the quality of your semen.
This can happen as a result of:
- an infection of your testicles
- testicular cancer
- testicular surgery
- a problem with your testicles you were born with (a congenital defect)
- when one or both testicles hasn't descended into the scrotum, the loose sac of skin that contains your testicles (undescended testicles)
- injury to your testicles
Some men choose to have a vasectomy if they don't want children or any more children.
It involves cutting and sealing off the tubes that carry sperm out of your testicles (the vas deferens) so your semen will no longer contain any sperm.
A vasectomy can be reversed, but reversals aren't usually successful.
Some men experience ejaculation problems that can make it difficult for them to release semen during sex (ejaculate).
Hypogonadism is an abnormally low level of testosterone, the male sex hormone involved in making sperm.
It could be caused by a tumour, taking illegal drugs, or Klinefelter syndrome, a rare syndrome where a man is born with an extra female chromosome.
Medicines and drugs
Certain types of medicines can sometimes cause infertility problems.
These medicines are listed below:
- sulfasalazine – an anti-inflammatory medicine used to treat conditions such as Crohn's disease and rheumatoid arthritis; sulfasalazine can decrease the number of sperm, but its effects are temporary and your sperm count should return to normal when you stop taking it
- anabolic steroids – are often used illegally to build muscle and improve athletic performance; long-term abuse of anabolic steroids can reduce sperm count and sperm mobility
- chemotherapy – medicines used in chemotherapy can sometimes severely reduce sperm production
- herbal remedies – some herbal remedies, such as root extracts of the Chinese herb Tripterygium wilfordii, can affect the production of sperm or reduce the size of your testicles
Illegal drugs, such as marijuana and cocaine, can also affect semen quality.
In the UK, unexplained infertility accounts for around 25% of cases of infertility. This is where no cause can be identified in either the woman or man.
If a cause for your fertility problems hasn't been identified, talk to your doctor about the next steps.
The National Institute for Health and Care Excellence (NICE) recommends that women with unexplained infertility who haven't conceived after 2 years of having regular unprotected sex should be offered IVF treatment.
The NICE guidance has more about unexplained infertility.
Find out more about fertility tests and how problems are diagnosed.
Page last reviewed: 14 February 2017
Next review due: 14 February 2020