Guidance

Carbon monoxide (CO) antenatal checks algorithm: summary

Updated 17 November 2025

This is an algorithm to help midwives and maternity staff diagnose environmental carbon monoxide (CO) poisoning in both smoking and non-smoking pregnant women and offer advice in the event of suspected exposure. This version is a summary.

The term ‘pregnant woman’, used throughout the guidance, should be taken to include people who do not identify as women, but who are pregnant or have given birth.

Algorithm

This algorithm is meant to serve as a guide. Regardless of the potential source of CO exposure, if you feel concerned about raised levels of CO and/or signs and symptoms of CO poisoning, please use your clinical judgement and follow the local trust’s policies.

Always record the exhaled CO level and any feedback given in the antenatal records.

The National Poisons Information Service provides information on clinical management through the TOXBASE® website, www.toxbase.org, or phone 0344 892 0111.

1. Offer information on CO and carry out CO breath test

Provide information on carbon monoxide, its risks, sources of exposure and carry out breath test.

2. Reading below 4ppm

Advise pregnant woman that she is unlikely to be exposed to CO.

3. Ask pregnant woman if she is a smoker

Ask pregnant woman if she is a smoker or has ever smoked, however infrequently.

4. You do not think pregnant woman is exposed to CO

She is unlikely to be exposed to CO, particularly if previous readings have been below 4ppm.

5. Reading of 4ppm or above

Advise that her reading is raised and the main source of exposure is usually smoking, however have a conversation about second-hand smoke and other environmental sources of CO.

6. Provide advice on environmental CO exposure

Advise on second-hand smoke, environmental sources, signs or symptoms and reducing CO levels.

7. Provide advice and refer to stop smoking services

Regardless of her CO level, express concern for her and her baby and use local referral systems. However, the raised level may not be completely due to smoking. Advise on second-hand smoke, environmental sources, signs or symptoms and reducing CO levels.

8. Reading of 10ppm or above for smokers or non-smokers

Inform her that her reading is high, and to protect her and her baby, you want to check if the level recorded is due to smoking, second-hand smoke and/or other environmental sources of CO.

9. Ask questions to investigate whether this could be a case of environmental CO poisoning. Are you suspicious?

• ‘Did you come here from your home, at what time did you leave and how did you travel here?’
• ‘Did you have your last smoke within the 4 hours preceding this test?’ (for smokers)

Positive responses to the following questions raise suspicion for both smokers and non-smokers:

• ‘Do you or cohabitees suffer from headache, dizziness, flu-like symptoms, weakness, drowsiness, confusion and/or nausea? If so, do symptoms improve when you are outside?’
• ‘Does your work involve exposure to smoke, fumes, or motor vehicle exhaust?’
• ‘Do you have gas, oil, or solid fuel appliances that have not been serviced recently, were newly installed, or are being used again after some time?’
• ‘Do you ever use your gas oven or stove for heating purposes as well as for cooking?’
• ‘Do you ever burn incense or ceremonial coals at home?’
• ‘Has there been any change in ventilation in your home recently?’

It is also important to ask: Do you have a working CO alarm in your home?

If no, recommend the installation of an audible CO alarm (BS EN 50291-1 compliant)

If yes, ask: Has it sounded recently?

10. Advise pregnant woman on steps to stop exposure and seek further advice

If symptomatic, refer the pregnant woman and/or cohabitees to emergency services indicating that symptoms may be related to CO poisoning, unless there is a specific local pathway. Advise her to contact the National Gas Emergency Service Line for advice, turn off fuel-burning appliances, open windows to ventilate the suspected space and do not return until advised to, and install (or by the landlord in rented properties) a BS EN 50291-1 compliant CO alarm (some also monitor low-level CO) for each combustion appliance.

Contacts

Emergency contacts

• Ambulance (severe symptoms): 999
• National Gas Emergency Helpline (concerns about fuel burning appliances): 0800 111 999

Useful contacts

• NHS (non-emergency line) (mild symptoms): 111
• Gas Safe Register (registered gas engineers): 0800 408 5500
• HETAS (registered solid fuel engineers): 01684 278170

Priority Services Register

Use this registration link to register.

Note: pregnant women may be eligible to register.