This section discusses the health implications of secondhand smoke exposure during pregnancy for the mother and foetus. It includes information on fertility, foetal growth and preterm delivery, foetal lung development, spontaneous abortion and stillbirth, and birth defects.
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, the influence of past (or any present) active maternal smoking (further discussed in Sections 3.7 and 3.8), and the potential damage to sperm from active paternal smoking. Nonetheless, evidence summarised below suggests that maternal secondhand smoke exposure during pregnancy is associated with an increased risk of pregnancy complications and poorer infant outcomes. A substantial body of this evidence is drawn from China, where the prevalence of active smoking is significantly lower among women compared to men.1 This evidence is particularly useful as the influence of maternal active smoking on such pregnancy complications and infant outcomes is reduced.
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 are most commonly used to detect exposure to secondhand smoke in foetuses of mothers who do not smoke. 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 secondhand smoke exposure may be associated with reduced fertility among women and a lower probability of conception. However, these effects may be difficult to separate from the effects of active smoking, and more research is required before these outcomes may be stated with certainty.3-5 Some studies have demonstrated that among 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
There is evidence to suggest that maternal secondhand smoke exposure during pregnancy increases the risk of preterm birth. The 2005 California Environmental Protection Agency report concluded that secondhand smoke exposure is a cause of preterm delivery, and may cause foetal growth restriction.3 The 2006 US Surgeon General’s report concluded that the evidence for secondhand smoke affecting preterm delivery is ‘suggestive’ of causality.9 Studies published since these reports have made similar conclusions,10-16 with a 2025 meta-analysis demonstrating that the odds of preterm birth increased by 21% for women who were exposed to secondhand smoke during pregnancy.10
Most studies examining the effects of secondhand smoke on foetal growth9,13,14,17-26 suggest that it increases the risk of small-for-gestational-age (SGA),19 low birth length,20 and low birthweight.9,20-26 The 2006 US Surgeon General’s report concluded that maternal exposure to secondhand smoke during pregnancy causes a small reduction in birth weight.9 Estimates of the mean decrease in weight calculated by various reviews range from about 30 g to 60 g, and secondhand smoke exposure has been demonstrated to increase the risk of low birthweight <3500 g by up to 22%.3,4,9,27 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 smoking, 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 study 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 infants of mothers exposed to secondhand smoke may be similar to those that cause lower birthweight in infants of women who actively smoke. Factors contributing to low birthweight may include preterm delivery, foetal growth restriction, or a combination of the two.9 The 2006 US Surgeon General’s report also cited research suggesting that lower birthweight may be the result of reduced oxygen to the foetus.9 Some studies have found that maternal secondhand smoke exposure increases the risk for maternal inflammation, reduced placental weight and foetal growth restriction.30-33
4.16.3 Foetal lung development
Despite clear evidence that active maternal smoking during pregnancy impairs foetal lung development,4,9,34 there is currently insufficient evidence to make the same conclusion for secondhand smoke. However, animal studies and some human studies indicate a possible effect of secondhand smoke exposure.
Animal studies suggest that maternal exposure to secondhand smoke also causes changes in foetal lung structure,9,34 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 neonatal intensive care unit, 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
The 2006 US Surgeon General’s report concluded that the evidence is inadequate to determine whether maternal exposure to secondhand smoke during pregnancy causes spontaneous abortion.9 A 2014 meta-analysis of 17 studies found no significant association between maternal exposure to secondhand smoke during pregnancy and miscarriage.39 However, a 2025 meta-analysis demonstrated that women exposed to secondhand smoke during pregnancy had a 29% increased risk of spontaneous abortion. Although, the authors of this study note that this estimate should be interpreted with caution due to unmeasured or residual confounding factors.40
There is some research to suggest that maternal exposure to secondhand smoke is associated with an increased risk of stillbirth. A 2011 meta-analysis of 4 studies demonstrated that women who were exposed to secondhand smoke during pregnancy were 23% more likely to experience a stillbirth compared to women who were not exposed.41 A 2019 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 substantial body of research has shown that maternal secondhand smoke exposure during pregnancy is associated with an increased risk of birth defects in offspring.41,43-46 A 2024 meta-analysis of 34 Chinese case-control studies found that maternal secondhand smoke exposure was associated with a 2.59-fold increased odds of any type of birth defect.44
Orofacial clefts are a common type of birth defect affecting the mouth and lip. Several meta-analyses have demonstrated that maternal secondhand smoke exposure is associated with an increased risk of orofacial clefts in offspring.43,45,46 A 2018 meta-analysis found that maternal secondhand smoke exposure during pregnancy was associated with a 1.9-fold increased risk of oral clefts.43 This study also found increased risks of digestive system, nervous system, and cardiovascular system malformations among offspring whose mothers were exposed to secondhand smoke during pregnancy.43 Several Chinese studies have found increased risks of circulatory system defects47 and congenital heart defects48,49 among offspring 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.50 Research in China also found a substantially increased risk of NTDs in the offspring of women exposed to secondhand smoke.51
4.16.6 Other effects
Pregnancy symptoms such as heartburn, abdominal pain and frequent urination may be more common among women exposed to secondhand smoke.52 Exposure to secondhand smoke during pregnancy may also be associated with reduced duration of breastfeeding among postpartum women.53,54 There is also some evidence to suggest that maternal secondhand smoke exposure during pregnancy is associated with an increased risk of pregnancy complications such as placental abruption,55 hypertensive disorders,56 and gestational diabetes.57,58
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 secondhand smoke exposure is associated with depressive symptoms,52,59-63 and poorer quality of life among pregnant women.23,64
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,65,66 delayed neurodevelopment,67 and psychomotor development,68 and lower academic achievement.69 Studies have also found an association between maternal secondhand smoke exposure and cerebral palsy,70 paediatric Crohn's disease,71 and childhood brain tumours.72 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).73
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References
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