Carbon monoxide testing in pregnancy

When you smoke, carbon monoxide – a poisonous gas – replaces some of the oxygen in your blood and affects your baby.

Carbon monoxide (CO) is a poisonous gas that you can't see or smell. 

It is released by faulty household appliances such as:

  • gas boilers  
  • gas cookers
  • wood, gas and coal fires
  • portable generators.

Cigarettes also produce carbon monoxide. Carbon monoxide is particularly harmful to developing babies.  

How does carbon monoxide affect my baby?

Carbon monoxide can restrict the essential oxygen supply to your baby. This can lead to serious complications, such as:

Find out more about how smoking can affect your pregnancy.

Carbon monoxide breath testing

Your midwife will do a carbon monoxide test at your first antenatal (‘booking’) appointment. The simple test will show how much of this gas you and your baby have been exposed to in the past 1-2 days.

How the carbon monoxide test works

You blow into a hand-held machine, called a CO monitor, which measures the level of carbon monoxide in your body.

Some machines can also work out how much of this gas is getting to your baby.

The more carbon monoxide you have breathed in, the more you will have in your body and the higher your reading will be.

The CO monitor gives your reading in ppm (parts per million), which means the number of parts of carbon monoxide in 1 million parts of air. If your reading is 4 ppm or higher, your midwife should refer you to NHS Stop Smoking Services.

All pregnant women and people who smoke, or have recently quit, will be referred for support to stop smoking.

If your first reading is high, you should also be offered a carbon monoxide test at all future antenatal appointments. Your CO readings will fall when you stop smoking.

Quitting smoking is hard but there is support available that is proven to work.  

What if I don’t smoke but my reading is high?

Your carbon monoxide level should be low if you don’t smoke. But there are other reasons why it might be higher than expected.

You may be breathing in harmful amounts of secondhand smoke.

You may be exposed to other household smoke, such as from a wood-burning stove.

You might be at risk of CO poisoning from a faulty gas appliance.  

The Health and Safety Executive has an advice line (0800 300 363) for information on gas safety. If you think you have been breathing in carbon monoxide from a faulty appliance, call the National Gas Emergency Service on 0800 111 999. You should also contact your GP.

Is there carbon monoxide in electronic cigarettes?

E-cigarettes do not produce tar or carbon monoxide, the 2 main toxins in cigarette smoke.  

The vapour from an e-cigarette contains some of the potentially harmful chemicals found in cigarette smoke, but at much lower levels. It is not known whether the vapour is harmful to a baby in pregnancy.

There has not been much research into the safety of e-cigarettes in pregnancy.  

If using an e-cigarette helps you to stop smoking, it is much safer for you and your baby than continuing to smoke. But licensed nicotine replacement therapy products such as patches and gum are the recommended option to help you stop smoking. Find out more about getting support to stop smoking.

NHS. Carbon monoxide poisoning. (Page last reviewed: 1 July 2022 Next review due: 1 July 2025)

NHS. Stop smoking in pregnancy. (Page last reviewed: 10 January 2023 Next review due: 10 January 2026)

National Institute for Health and Care Excellence (2021). Tobacco: preventing uptake, promoting quitting and treating dependence. NICE guideline NG209. 

Review dates
Reviewed: 23 November 2023
Next review: 23 November 2026