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Carbon monoxide testing

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 colourless, odourless and tasteless poisonous gas. You can't see it or smell it, but it is in cigarette smoke. CO is also in faulty gas boilers and car exhaust fumes.

How does carbon monoxide affect my baby?

When you smoke a cigarette (this includes manufactured cigarettes or roll-ups), CO replaces some of the oxygen going into your lungs and gets into your bloodstream. This prevents the red blood cells from carrying oxygen around your body and to your baby.

It is dangerous because it deprives the baby of oxygen, slows the baby’s growth and development, and increases the risk of miscarriage, stillbirth and sudden infant death syndrome (SIDS).

Carbon monoxide breath testing

As part of your routine care, your midwife should carry out a carbon monoxide test at your first booking appointment and at subsequent visits. This simple test will show how much of this gas you and your baby have been exposed to. 

How the CO test works:

  • You blow into a hand-held machine, called a CO monitor, which measures the level of CO in your body.
  • Some machines can also work out how much CO is getting to your baby.
  • The more CO you have inhaled, the higher your CO reading will be. This is because the more smoke you have inhaled, the more CO you will have in your body.

This test can also show whether you’re inhaling harmful amounts of secondhand smoke.

If you see the results of the CO test, the reading is measured in COppm, which means the number of CO molecules in 1 million parts of air. Women with a CO test reading of 3ppm or higher may be referred to NHS Stop Smoking Services.

When you stop smoking, your CO readings will fall, showing that you and your baby are becoming free from this poisonous gas. If your reading is high and you do not smoke, you might be at risk of CO poisoning from a faulty appliance and you should call the Health and Safety Executive gas safety line on 0800 300 363.

Is there carbon monoxide in electronic cigarettes?

No. Electronic cigarettes do not contain carbon monoxide or many of the other harmful gases found in cigarettes. If you only are using electronic cigarettes, you will not have a higher reading in a CO test.

NHS Choices. Stop smoking in pregnancy. (Page last reviewed: 07/11/2019. Next review due: 07/11/2022)

NICE (2010). Smoking: stopping in pregnancy and after childbirth. National Institute for health and care excellence

Farsalinos KE, Polosa R (2014). Safety evaluation and risk assessment of electronic cigarettes as tobacco cigarette substitutes: a systematic review. Therapeutic advances in drug safety. 2014 Apr;5(2):67-86. doi: 10.1177/2042098614524430.

Review dates
Reviewed: 23 January 2020
Next review: 23 January 2023

This content is currently being reviewed by our team. Updated information will be coming soon.