4.16 Secondhand smoke and pregnancy

Last updated: June 2022
Suggested citation: Greenhalgh, EM, Campbell MA, Ford C, & Winstanley MH. Ch 4. The health effects of secondhand smoke. 4.16 Secondhand smoke and pregnancy. In Greenhalgh, EM, Scollo, MM and Winstanley, MH [editors].  Tobacco in Australia: Facts and issues. Melbourne: Cancer Council Victoria; 2022. Available from:  http://www.tobaccoinaustralia.org.au/chapter-4-secondhand/4-16-secondhand-smoke-and-pregnancy

 

Determining the health impacts of secondhand smoke exposure during pregnancy is challenging because it is difficult to differentiate between the impacts of maternal secondhand smoke exposure, potential damage to sperm from paternal active smoking, and the influence of past maternal active smoking. This is particularly the case for rare conditions such as birth defects or childhood cancers. Nonetheless, evidence summarised below suggests that exposure to secondhand smoke during pregnancy is associated with an increased risk of pregnancy complications and poorer outcomes for infants. Among other risks, exposure to secondhand smoke can increase pregnant women’s exposure to heavy metals, which are particularly harmful to pregnant women and foetuses.1

Biomarkers to determine neonatal exposure to tobacco smoke prior to birth can be measured in cord blood, or neonatal urine, hair, nails, amniotic fluid and meconium (the first faecal matter passed by the baby after birth). When this is studied these samples are usually collected at or soon after birth. Metabolites of nicotine, in particular cotinine, in cord blood and meconium is most commonly used to detect exposure to secondhand smoke in foetuses of non-smoking mothers.  Biomarkers in cord blood indicate exposure to tobacco smoke in the few days before delivery, whereas biomarkers in meconium accumulate over several months of gestation.2

4.16.1 Fertility

Some evidence suggests that exposure to secondhand smoke may be associated with reduced fertility or fecundity (rate of conception) in women, however these effects may be difficult to separate from the effects of active smoking on the partner’s fertility.3-5 More research is required before these outcomes may be stated with certainty.4 In women undergoing IVF treatment, exposure to secondhand smoke may lead to a reduced chance of successful implantation.6, 7  Similarly, exposure to secondhand smoke may lower the chance of conception for women undergoing ovulation induction.8

4.16.2 Foetal growth and preterm delivery

Studies have shown that maternal secondhand smoke exposure is associated with preterm birth,9-14 and while the strength of conclusions have differed, major reviews have also suggested a relationship between secondhand smoke exposure and preterm delivery. The California Environmental Protection Agency report (2005) concluded that secondhand smoke is a cause of preterm delivery, and may cause intrauterine growth retardation.3 The US Surgeon General’s report (2006) concluded that the evidence for preterm delivery is mixed but ‘suggestive’ of causality.15 A more recent review found that the strongest evidence for an association between secondhand smoke exposure and preterm birth comes from studies that use both self-report and biomarker measurements, which suggest that high levels of exposure may increase the risk.16 Some evidence also suggests that the introduction of smokefree legislation is associated with reductions in preterm births (see also Section 15.9), though this may also be attributable to reductions in smoking among pregnant women.16

Most studies examining the effects of secondhand smoke on foetal growth11, 12, 15, 17-26 suggest that it increases the risk of small-for-gestational-age (SGA),19 low birth length,20 and low birthweight.15, 20-26 The US Surgeon General concluded in 2006 that maternal exposure to secondhand smoke during pregnancy causes a small reduction in birth weight.15 Estimates of the mean decrease in weight calculated by various reviews range from around 30 g to 60 g, and secondhand smoke exposure has been demonstrated to increase the risk of low birthweight <3500g by up to 22%.3, 4, 15, 27 One study examining women exposed to secondhand smoke who carried specific genotypes related to metabolising enzymes found that these women delivered babies with mean birthweight decrements of around 160 g and 200 g.4 This suggests there may be a sub-group of women whose foetuses are more susceptible to the effects of maternal secondhand smoke exposure.3 The level of exposure may also be important, with one study finding that, for both active and passive smokers, a plasma cotinine level (which indicates the level of nicotine in the blood) of ≥3.03 ng/mL was associated with an increased risk of SGA.28 Another found evidence of a relationship between exposure to more than 22 cigarettes a day and reduced head circumference at birth and head circumference gain over the first six months of life.29

Mechanisms for lower birthweight in babies of non-smoking mothers exposed to secondhand smoke may be similar to those that cause lower birthweight in infants of active smokers. Factors contributing to low birthweight may include preterm delivery, intrauterine growth retardation, or a combination of the two.15 The US Surgeon General’s report (2006)  also cited research suggesting that lower birthweight may be the result of reduced oxygen to the foetus.15 Some studies have found that secondhand smoke exposure of the pregnant mother increases the risk for maternal inflammation, reduced placental weight and foetal growth restriction.30-33

4.16.3 Lung development in the unborn child

There is clear evidence that smoking by a pregnant mother impairs foetal lung development.4, 15, 34 Animal studies suggest that maternal exposure to secondhand smoke also causes changes in foetal lung structure,15, 34 including direct effects on alveolar development in foetal lungs.35 One study on pregnant sheep indicated that maternal exposure to secondhand smoke may decrease blood flow and increase vascular resistance in the foetal lungs. In this study secondhand smoke also appeared to reduce the normal dilation of blood vessels in the foetal lungs in response to increased oxygen. These effects are associated with a marked decrease in oxygen within the foetus, and may affect lung circulation at birth.36 A study in Iran found that newborn infants of women exposed to secondhand smoke during pregnancy were more likely to be admitted to the NICU, with one of the causes being asphyxia (insufficient oxygen).37 Secondhand smoke exposure may also affect later lung function, with a Taiwanese study showing that prenatal exposure to tobacco smoke (via the father’s smoking) was associated with epigenetic changes that are related to childhood asthma.38

4.16.4 Spontaneous abortion (miscarriage) and stillbirth

Findings on an association between maternal exposure to secondhand smoke and an increased rate of miscarriage are mixed, though some suggest an increased risk.3, 4, 15, 39 One study has suggested that exposure to secondhand smoke during childhood could increase a woman’s risk of miscarriage later in life.40 A meta-analysis published in 2014 concluded that secondhand smoke exposure during pregnancy increased the risk of miscarriage by 11%.41

A meta-analysis published in 2011 based on 19 studies demonstrated that maternal secondhand smoke exposure was associated with an increased risk of stillbirth,39 and a more recent study in Egypt also found an association between secondhand smoke exposure and stillbirth, particularly among women aged 25–29 years.42

4.16.5 Birth defects

A number of meta-analyses have shown that maternal secondhand smoke exposure is associated with an increased risk of birth defects.39, 43 A large study in China published in 2022 estimated that pregnant women exposed to secondhand were 58% more likely to have infants with overall birth defects.44 A 2015 meta-analysis found maternal secondhand smoke exposure was associated with a 1.5-fold increase in the risk of orofacial clefts, similar to the increased risk demonstrated for active smoking.45 More recently, a meta-analysis published in 2018 found that secondhand smoke exposure during pregnancy was associated with a 1.9-fold increased risk of oral clefts, as well as increased risks of digestive system (1.2-fold), nervous system (1.7-fold), and cardiovascular system (2.1-fold) malformations.43 Several studies in China have found increased risks of circulatory system defects44 and congenital heart defects46, 47 among infants whose mothers were exposed to secondhand smoke during pregnancy.

A 2018 meta-analysis that examined the relationship between secondhand smoke and neural tube defects (NTDs; birth defects of the brain, spine, or spinal cord) concluded that exposure to secondhand smoke clearly increased the risk of NTDs, with findings suggesting the risk may be greater than for maternal smoking during pregnancy.48 Research in China also found a substantially increased risk of NTDs in babies of women exposed to secondhand smoke.49

4.16.6 Other effects

There is a robust relationship between smoking and poorer mental health (see Section 9A.3) , and emerging evidence suggests that secondhand smoke exposure is associated with depressive symptoms, psychological distress and stress (see Section 4.14). Studies have also found that exposure to secondhand smoke is associated with depressive symptoms50-54 and poorer quality of life23, 55 among pregnant women. Pregnancy symptoms such as heartburn, abdominal pain and frequent urination may also be more common among women exposed to secondhand smoke.54 Exposure to secondhand smoke during pregnancy is also associated with reduced duration of breastfeeding among postpartum women.56, 57

4.16.7 Maternal secondhand smoke exposure and infant and child health

Exposure to secondhand smoke in childhood is associated with an increased risk of many conditions in childhood (see Section 4.17). Evidence suggests that some of these risks may also be increased for children of women exposed to secondhand smoke during pregnancy, such as the risk of attention deficit hyperactivity disorder (ADHD) symptoms,58 delayed neurodevelopment59 and psychomotor development,60 and lower academic achievement.61 Studies have also found an association between maternal secondhand smoke exposure and cerebral palsy62 and paediatric Crohn's disease.63 One study of foetal autopsies suggested that infants of women exposed to secondhand smoke were more likely to have lesions in the walls of the foetal artery and adjoining vessels, which reflect the initial stages of atherosclerosis (narrowing of the arteries by fatty deposits).64

 

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