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 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:
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).
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 (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.
The side effects of some types of medicines and drugs can affect your fertility.
Illegal drugs, such as marijuana and cocaine, can seriously affect fertility and make ovulation more difficult.
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:
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:
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.
Certain types of medicines can sometimes cause infertility problems.
These medicines are listed below:
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.