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1332 pages added, reviewed or updated during the last month (last updated: 27/2/2021)

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carbon monoxide testing in pregnancy

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Smoking cessation advice during pregnancy should be undertaken by mdwives (at first maternity booking and subsequent appointments)

What action should the midwife take?

  • assess the woman's exposure to tobacco smoke through discussion and use of a CO test
    • explain that the CO test will allow her to see a physical measure of her smoking and her exposure to other people's smoking
    • ask her if she or anyone else in her household smokes. To help interpret the CO reading, establish whether she is a light or infrequent smoker
    • other factors to consider include the time since she last smoked and the number of cigarettes smoked (and when) on the test day. (Note: CO levels fall overnight so morning readings may give low results.)

  • provide information (for example, a leaflet) about the risks to the unborn child of smoking when pregnant and the hazards of exposure to secondhand smoke for both mother and baby. Information should be available in a variety of formats

  • explain about the health benefits of stopping for the woman and her baby. Advise her to stop - not just cut down

  • explain that it is normal practice to refer all women who smoke for help to quit and that a specialist midwife or adviser will phone and offer her support. (Note: a specialist adviser needs to offer this support to minimise the risk of her opting out.)

  • refer all women who smoke, or have stopped smoking within the last 2 weeks, to NHS Stop Smoking Services
    • also refer those with a CO reading of 7 ppm or above. (Note: light or infrequent smokers should also be referred, even if they register a lower reading - for example, 3 ppm.) If they have a high CO reading (more than 10 ppm) but say they do not smoke, advise them about possible CO poisoning and ask them to call the free Health and Safety Executive gas safety advice line on: 0800 300 363 .


  • a carbon monoxide (CO) test is an immediate and non-invasive biochemical method for helping to assess whether or not someone smokes. However, it is unclear as to what constitutes the best cut-off point for determining smoking status
    • some suggest a CO level as low as 3 parts per million (ppm), others use a cut-off point of 6-10 ppm
      • it is important to note that CO quickly disappears from expired breath (the level can fall by 50% in less than 4 hours). As a result, low levels of smoking may go undetected and may be indistinguishable from passive smoking
      • conversely, environmental factors such as traffic emissions or leaky gas appliances may cause a high CO reading - as may lactose intolerance. When trying to identify pregnant women who smoke, it is best to use a low cut-off point to avoid missing someone who may need help to quit


Last reviewed 01/2018