If chlamydia isn't treated, it can sometimes spread and cause potentially serious problems.

Pelvic inflammatory disease (PID)

In women, chlamydia can spread to the womb, ovaries or fallopian tubes. This can cause a condition called pelvic inflammatory disease (PID).

PID can cause a number of serious problems, such as:

• difficulty getting pregnant or infertility
• persistent (chronic) pelvic pain
• an increased risk of ectopic pregnancy (where a fertilised egg implants itself outside the womb)

The symptoms of PID are generally similar to the symptoms of chlamydia, including discomfort or pain during sex, pain during urination, and bleeding between periods and after sex.

PID is usually treated with a 2-week course of antibiotics. The risk of experiencing problems such as infertility is lower if it's treated early, so it's important to seek medical advice as soon as possible if you have symptoms of the condition.

Pregnancy complications

If you have chlamydia that's not treated while you're pregnant, there's a chance you could pass the infection on to your baby. If this happens, your baby may develop an eye infection (conjunctivitis) and lung infection (pneumonia).

If your baby has symptoms of these conditions, your midwife or GP can arrange for a test to check for chlamydia, and antibiotics can be used to treat the infection.

Untreated chlamydia in pregnancy may also increase the risk of problems such as premature labour and birth (before 37 weeks of pregnancy) or your baby being born with a low birthweight.

Inflammation of the testicles

In men, chlamydia can spread to the testicles and epididymis (tubes that carry sperm from the testicles), causing them to become painful and swollen. This is known as epididymitis or epididymo-orchitis. This is very rare.

The inflammation is usually treated with antibiotics. If it's not treated, there's a possibility it could affect your fertility.

Reactive arthritis

Chlamydia is the most common cause of sexually acquired reactive arthritis (SARA). This is where your joints, eyes or urethra (the tube that passes urine out of the body) become inflamed, usually within the first few weeks after having chlamydia.

It can affect women who have had chlamydia but is more common in men.

There's currently no cure for SARA, but most people get better in a few months. In the meantime, treatment with non-steroidal anti-inflammatory drugs (NSAIDs) such as ibuprofen can help relieve the symptoms.