Test your breath

Test your breath: information for pregnant women

Why carbon monoxide screening matters

Carbon monoxide (CO) is a poisonous gas which you can’t see or smell but which is dangerous to you and your baby. Exposure can prevent oxygen reaching your baby, slow its growth and development, and can result in miscarriage, stillbirth and sudden infant death.

Exposure to CO can be measured through a quick and simple breath test provided by your midwife. The test will give you a number which measures the amount of carbon monoxide in parts per million (PPM).


Your recent level of exposure to carbon monoxide is low

This shows little exposure to carbon monoxide in the last 24-48 hours


You have had some recent exposure to carbon monoxide

This suggests you have had recent exposure to carbon monoxide and this may be of concern


Exposure to carbon monoxide is usually from one of three ways:

  • Cigarette smoke – this is the most common reason
  • Faulty or poorly ventilated cooking or heating appliances
    (this includes gas, coal, wood and parafin appliances)
  • Faulty car exhausts

If you or anyone in your home smokes, this is the most likely explanation for the high reading

Reducing your exposure to cigarette smoke is the most important thing you can do for you and your baby’s health.

This may be by quitting smoking yourself or reducing your exposure from others, by asking smokers not to smoke in the home, car or in front of you.

Once you stop, the carbon monoxide clears from your bloodstream and that of your baby, allowing a good flow of oxygen to support their growth and development.

Your midwife can discuss options to help you quit, and will refer you to a local stop smoking service. To find out more about the free support available, call the NHS Smokefree helpline on 0300 123 1044 (England only). Or visit the Smokefree website at www.smokefree.nhs.uk

If you are not usually exposed to cigarette smoke, but you have a reading of 4 or more, you may have been exposed to carbon monoxide through faulty heating or cooking appliances

We strongly recommend that you get expert help from the National Gas Emergency Service Line 0800 111 999 or go to cadentgas.com/emergencies/smell-gas

It is important to check that your heating and cooking appliances are safely installed. You may wish to buy a carbon monoxide alarm that will detect low levels of carbon monoxide in your home.

More info and links

  • To get free week-by-week pregnancy tips you can trust from NHS experts visit www.nhs.uk/start4life
  • Download the NHS Smokefree app from the App Store or Google Play to help you quit smoking and start breathing easier
  • The Tommy’s Healthy Pregnancy Tool provides evidence based advice about having a healthy pregnancy
  • The free Baby Buddy app developed by Best Beginnings provides trusted, evidence-based information and self-care tools to help parents build their knowledge and confidence during the transition to parenthood and throughout the early stages of parenting
  • The Lullaby Trust provides information and advice to help you have a healthy pregnancy and a healthy baby

Other languages

This information is also available in the following languages: Bulgarian | Somalian | Romanian | Lithuanian | Urdu | Turkish | Punjabi | Portuguese | Polish | Greek)