Ectopic pregnancy - Causes of ectopic pregnancy

In the early stages of pregnancy, an egg is released from one of your ovaries into one of your fallopian tubes, where it is fertilised by sperm.

Each fallopian tube is about 10cm (4 inches) long and lined with millions of moving, hair-like structures called cilia. In a normal pregnancy, the cilia push the fertilised egg along the tube and into the womb, where the egg implants itself into the womb's lining (endometrium) and develops into a baby.

However, if the fallopian tube has been damaged (for example, if there is a blockage or narrowing of the tube), the cilia may not be able to move the egg to the womb, and the pregnancy may develop in the fallopian tube.

Common risk factors

Some of the most common risk factors for an ectopic pregnancy are discussed below.

Pelvic inflammatory disease

Pelvic inflammatory disease (PID) is a bacterial infection of the female reproductive system. Most cases of PID are caused by an infection in the vagina or the neck of the womb (cervix) that has spread to the reproductive organs higher up.

Many different types of bacteria can cause PID, but most cases are due to a chlamydia infection – this is a type of sexually transmitted infection that can be spread during unprotected sex.

Chlamydia often exhibits no noticeable symptoms, so women may not know they are infected. However, the bacteria can cause inflammation of the fallopian tubes, which is known as salpingitis. Salpingitis leads to a four-fold increase in the risk of having an ectopic pregnancy.

Previous history

Having a previous history of ectopic pregnancy means you have an increased risk of having one in the future.

Depending on the underlying factors, the risk of having another ectopic pregnancy is somewhere between 1 in 10 and 1 in 4.

Previous surgery

If you have ever had surgery that involved your fallopian tubes, you have an increased risk of having an ectopic pregnancy. Types of surgery known to increase your risk include:

• female sterilisation (a type of surgery known as tubal ligation or "tying the tubes") – in around 1 in 200 cases surgery fails, the woman becomes pregnant and can result in an ectopic pregnancy
• earlier surgery to remove a previous ectopic pregnancy

Fertility treatments

Taking medication to stimulate ovulation (the release of an egg) can increase the risk of ectopic pregnancy by around four-fold.

The type of fertility treatment known as in-vitro fertilisation (IVF) is not always successful and can accidentally result in an ectopic pregnancy.

This occurs in around 1 in 22 cases of IVF.

Contraception

The intrauterine device (IUD) and the intrauterine system (IUS) are very effective in preventing pregnancy – the success rate is estimated to be around 99 out of 100 cases. But if a pregnancy does occur when using these types of contraception, it is more likely to be an ectopic pregnancy than a normal pregnancy.

There is also a risk that if you take emergency contraception and it fails to work, any subsequent pregnancy could be an ectopic pregnancy.

Other risk factors

Other potential risk factors for an ectopic pregnancy include:

• structural problems – ectopic pregnancy is more likely if you have an abnormally shaped fallopian tube
• smoking – smokers are twice as likely to have an ectopic pregnancy than non-smokers
• being aged over 35