Background

Smoking in pregnancy is associated with significant harmful effects to unborn infants during the pregnancy period and their childhood. This association has been established, including through the ‘gold standard’ of random clinical trials, for decades¹. There are a wide range of harmful effects of smoking in pregnancy including reduced birth weight, spontaneous pre-term birth, placental abruption, ectopic pregnancy and many more. It is therefore universally agreed across clinicians and public health professionals, as is the case with the general population, that there is no safe level of smoking in pregnancy.

Smoking in the general population has reduced substantially from as high as one in two in the middle of the 20th century to approximately one in six today². Despite the well documented risks both to the mother and infant, smoking during pregnancy has been estimated to be as high as one in eight³ while smoking rates at delivery of the baby are approximately one in ten in the UK². Educating women of the harms of smoking before and during pregnancy and encouraging them to stop is therefore of paramount importance for the mother’s and infant’s short and long-term health and life chances.

Health effects of smoking in pregnancy

Still birth (fetal loss at 20 weeks gestation or later) is a devastating complication of smoking in pregnancy. A recent study assessed all published evidence on this issue. This found that women who smoked during pregnancy had a 47% higher risk of a still birth than non-smokers. Importantly, the risk of smoking upon still birth appears to be ‘dose dependent’. This means that the risk of stillbirth increases as the smoking amount also increases. In women who smoked between one and nine cigarettes a day during pregnancy, the risk of still birth was 9% higher than non-smokers. However, in those who smoke ten or more cigarettes a day, this risk increased substantially to a 52% higher risk than non-smokers⁴. Stopping smoking during pregnancy can reduce this risk substantially⁵.

The association between smoking and other complications is also strong. For example, smoking during pregnancy increases the risk of infant mortality (the death of a child under the age of one year) by 40%⁶. Exposure to smoke during pregnancy is also associated with phsycological conditions later in childhood and adulthood which can seriously affect the child’s educational attainment. Studies suggest an increased risk of attention and hyperactivity problems, disruptive behaviour⁷ and a detrimental effect on the child’s future educational performance in children born to mothers who smoked during pregnancy⁸.

Passive smoking in pregnancy

Passive smoking (where you are exposed to other people’s tobacco smoke) is also associated with harm to the infant. Infants in the womb exposed to passive smoking have a higher risk of pre-term birth, low birth weight and a range of other health conditions. The 2007 smoking ban in enclosed places helped reduce the risk of passive smoking but pregnant women (and all of the population) should be aware of the risks that passive smoking carries.

Carbon monoxide test

Carbon monoxide is a poisonous gas which can result in reducing the amount of oxygen delivered to a baby in the womb. Smoking tobacco increases the carbon monoxide (CO) content of your blood. The level of CO in the blood of a non-smoker depends on background levels in the air, but it is usually between 0 and 8 parts per million. The level of CO for a smoker is typically much higher.

It is important to detect carbon monoxide levels in pregnant women’s blood as this can lead to serious adverse effects upon the unborn infant such as still birth and brain damage. Carbon monoxide levels are tested in the pregnant women at their antenatal appointment. This is a breath test and is useful for all pregnant women to measure the level of exposure to carbon monoxide that could be from smoking, passive smoking or even from faulty gas appliances, exposure to other pollutants or through other medical conditions. This can then be monitored throughout the pregnancy.

Summary

Smoking in pregnancy can cause serious, harmful effects upon the baby both during pregnancy and later in life. Women planning a pregnancy should make smoking cessation a priority to improve their own health and give their child the best start in life. There are a number of evidence based, effective smoking cessation methods which can significantly increase the likelihood of successfully quitting smoking which are discussed in an accompanying article. The overarching medical advice is that smoking in pregnancy is not safe.

References

1. Sexton M, Hebel JR. A clinical trial of change in maternal smoking and its effect on birth weight. Jama 1984;251:911-5.
2. Public Health England. Fingertips Public Health Profiles 2017. http://fingertips.phe.org.uk/search/smoking#page/11/gid/1/pat/6/par/E12000004/ati/102/are/E06000015
3. NHS Information. The Infant Feeding Survey. 2010. http://www.hscic.gov.uk/
4. Marufu TC, Ahankari A, Coleman T, Lewis S. Maternal smoking and the risk of still birth: systematic review and meta-analysis. BMC Public Health 2015;15:239.
5. McCowan LM, Dekker GA, Chan E, et al. Spontaneous preterm birth and small for gestational age infants in women who stop smoking early in pregnancy: prospective cohort study. Bmj 2009;338:b1081.
6. Department of Health. Review of the health inequalities infant mortality PSA target. London: Department of Health 2007.
7. Button TM, Maughan B, McGuffin P. The relationship of maternal smoking to psychological problems in the offspring. Early human development 2007;83:727-32.
8. Batstra L, Hadders-Algra M, Neeleman J. Effect of antenatal exposure to maternal smoking on behavioural problems and academic achievement in childhood: prospective evidence from a Dutch birth cohort. Early human development 2003;75:21-33.

 

Main points

• Smoking in pregnancy may cause reduced birth weight, spontaneous pre-term birth, placental abruption, ectopic pregnancy and other harmful effects
• There is no safe level of smoking in pregnancy
• A study has found that women who smoked during pregnancy had a 47% higher risk of a still birth than non-smokers
• Women who smoked between one and nine cigarettes a day during pregnancy, had a 9% higher chance of suffering a still birth than non-smokers.
• Women who smoked ten or more cigarettes a day, had a 52% higher risk than non-smokers
• Smoking during pregnancy increases the risk of infant mortality (the death of a child under the age of one year) by 40%
• Children exposed to smoke while in the womb had an increased risk of attention and hyperactivity problems, disruptive behaviour studies suggest.
• Smoking during pregnancy had a detrimental effect on children’s future educational performance
• Infants in the womb whose mothers were exposed to passive smoking have a higher risk of pre-term birth, low birth weight and a range of other health conditions.