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Date of issue: April 2024, Version: 3

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A corresponding patient information leaflet on EXPOSURE TO CARBON MONOXIDE IN PREGNANCY is available.

Carbon monoxide (CO) is a colourless, odourless, and tasteless gas produced by incomplete combustion of carbon-containing products. Common sources of exposure include faulty heating appliances (gas and solid fuel boilers), BBQs being used inside homes, caravans and tents, car exhausts, open fires, kerosene stoves and cigarette smoke.

Following maternal exposure, CO crosses the placenta and may reach higher concentrations in the fetus than in the mother. The elimination half-life in the fetus may be up to 4-5 times longer than in the mother. High maternal carboxyhaemoglobin (COHb) concentrations may confer a greater risk to both the mother and fetus but the correlation between maternal COHb blood concentrations and the clinical severity of maternal poisoning is weak.

Adverse outcomes have been reported after acute CO poisoning in pregnancy. Fetal and neonatal death, congenital malformations and neurological problems have all occurred in association with reported moderate-to-severe (loss of consciousness/coma) maternal toxicity. An increased risk of adverse outcomes cannot be excluded in the absence of maternal toxicity, for example, following low level chronic exposure. In the absence of good quality evidence, pragmatic advice would be to perform fetal investigations/monitoring and potentially treat asymptomatic mothers (i.e. those with low level chronic exposure).

Some studies investigating chronic environmental in utero exposure to CO have reported associations with preterm delivery, low birth weight, congenital malformations, sudden infant death and neurodevelopmental problems. However, these studies often include women exposed through tobacco smoking and, as such, data are likely confounded. Please refer to the UKTIS monograph on ‘Use of tobacco in pregnancy’ for further information on exposure to CO via smoking.

Maternal toxicity is likely to be a major determinant of risk to the fetus. UK guidance on antenatal testing for CO exposure during pregnancy is available online from Public Health England (PHE) (click here).

There are no published guidelines on the management of CO poisoning during pregnancy. Initial management of the poisoned pregnant patient should be the same as for the non-pregnant patient. Administration of high concentration supplemental oxygen therapy as soon as possible has been advocated if CO exposure is associated with an abnormal fetal heartbeat, or where there has been a delay in presentation following significant exposure associated with maternal symptoms that have resolved spontaneously, due to the difference between maternal and fetal pharmacokinetics of CO. There is no evidence-based guidance about the duration pregnant women should receive high concentration supplemental oxygen. Due to fetal pharmacokinetics, UKTIS pragmatically recommend five times the non-pregnant treatment duration required to reduce the maternal COHb to normal.

For current guidelines on the general management of CO poisoning consult TOXBASE or contact UKTIS. Where exposure to CO has occurred, even in the absence of maternal toxicity, enhanced fetal monitoring may be warranted. Discussion with UKTIS is recommended for all cases of CO poisoning in pregnancy.

This is a summary of the full UKTIS monograph for health care professionals and should not be used in isolation. The full UKTIS monograph and access to any hyperlinked related documents is available to NHS health care professionals who are logged in.

If you have a patient with exposure to a drug or chemical and require assistance in making a patient-specific risk assessment, please telephone UKTIS on 0344 892 0909 to discuss the case with a teratology specialist.

If you would like to report a pregnancy to UKTIS please click here to download our pregnancy reporting form. Please encourage all women to complete an online reporting form.

Disclaimer: Every effort has been made to ensure that this monograph was accurate and up-to-date at the time of writing, however it cannot cover every eventuality and the information providers cannot be held responsible for any adverse outcomes of the measures recommended. The final decision regarding which treatment is used for an individual patient remains the clinical responsibility of the prescriber. This material may be freely reproduced for education and not for profit purposes within the UK National Health Service, however no linking to this website or reproduction by or for commercial organisations is permitted without the express written permission of this service. This document is regularly reviewed and updated. Only use UKTIS monographs downloaded directly from to ensure you are using the most up-to-date version.