Smoking during pregnancy is the most common preventable risk factor for pregnancy complications.1 It is also associated with poorer outcomes for infants and children—see Section 3.8. For a discussion of interventions to help pregnant women quit smoking, see Section 7.11. In Australia in 2017, one in 10 women (9.9%) who gave birth smoked at some time during their pregnancy. Rates of smoking were higher in the first 20 weeks of pregnancy (9.5%) than after 20 weeks of pregnancy (7.3%). The likelihood of smoking during pregnancy was higher among teenagers, women in disadvantaged circumstances and Aboriginal and Torres Strait Islander women1 (see Section 1.10).
Many of the constituents of cigarette smoke are potentially toxic to the developing foetus, including lead, nicotine, cotinine, cyanide, cadmium, mercury, carbon monoxide and polycyclic aromatic hydrocarbons (PAHs).2, 3 Carbon monoxide (CO) reduces the oxygen supply to the foetus, leading to hypoxia (insufficient oxygen). CO binds to haemoglobin with an affinity 200 times that of oxygen, and also has an inhibiting effect on the release of oxygen to the cells. Chronic mild hypoxia of foetal tissue can persist for five or six hours after the woman has stopped smoking.3 Cadmium, a carcinogen, which accumulates in the placenta and has been detected in umbilical cord blood, is associated with a reduction in foetal capillary volume. 3 Nicotine is found in foetal blood, amniotic fluid and breast milk,2 and has short- and long-term effects likely to be related to several adverse pregnancy outcomes.3 Studies suggest that exposure to PAHs may disrupt hormones, alter enzyme levels or activity, and damage DNA, which, if not repaired, can lead to cell death, cancer or foetal abnormalities.3
There are various mechanisms through which smoking may affect the pregnancy and the development of the foetus. Both smoking and nicotine by itself change hormone patterns, affecting the pregnancy outcome and the endocrine profile of the infant. Smoking and nicotine affect the functioning and structure of the oviduct (fallopian tube) in ways that could impair fertility (see Section 3.6) and complicate the pregnancy. Smoking disturbs the development of the placenta, disrupting the implantation process and interfering with the transformation of the uterine spiral arteries. Studies consistently show thickening of the villous membrane of the placenta in smokers, which decreases the ability of nutrients to diffuse through the placenta. Smoking and nicotine impair amino acid transport across the placenta, which the foetus needs to make proteins. Nicotine may decrease the pumping of fluid across the placenta, leading to lower oxygen levels in the foetus and acidosis (excessive acid in the blood and tissues).3, 4 Nicotine can alter embryonic movements that are important in the early development of the organs. Consistent evidence shows that smoking can affect the development of the foetal lung and brain. Smoking a cigarette temporarily increases maternal heart rate and blood pressure and decreases foetal heart rate variability (a measure of the infant’s wellbeing). 3 Studies show a decrease in foetal movement for at least an hour after smoking one cigarette, consistent with a reduction of oxygen to the foetus.5
Smoking may present further risks to the pregnancy by increasing the woman’s risk of infectious disease and altering the inflammatory response of her immune system. Smokers have lower levels of micronutrients that play a vital role in the health of the pregnancy, such as zinc (in cord blood) and vitamin C. Vitamin C is important for immune function and the formation of collagen. Genetic variation in enzymes that metabolise chemicals from tobacco smoke mediate the risk of adverse pregnancy outcomes. Several proposed mechanisms for the effects of smoking on pregnancy are presented in more detail in the US Surgeon General’s report of 2010.3
3.7.1 Spontaneous abortion
Smoking during pregnancy is associated with spontaneous abortion or miscarriage (the involuntary termination of a pregnancy prior to 20 weeks of gestation).3, 6-8 Because many miscarriages occur too early to be recognised and confirmed, and miscarriage can be caused by a number of preconditions, exposures or events, spontaneous abortions are difficult to study. However there are multiple ways in which smoking could potentially increase the risk for miscarriage. Proposed mechanisms include placental insufficiency, chronic reduced oxygen to the foetus, and direct toxic effects of constituents of cigarette smoke.3 Tobacco or nicotine may also affect the quality of the egg and embryonic development.6
3.7.2 Ectopic pregnancy
Smoking during pregnancy causes ectopic pregnancy (the implantation of a fertilised egg occurring outside the uterus, usually in the fallopian tubes).6 Nicotine slows down the movement of the fertilised ovum in the fallopian tubes, and impairment of oviduct function can lead to ectopic pregnancy.3 Smokers also have a higher risk of developing pelvic inflammatory disease, which is associated with ectopic pregnancy.2
3.7.3 Complications of pregnancy
Smoking affects the healthy development and function of the umbilical cord and placenta, and causes abnormalities or insufficiencies that can lead to serious complications for the success of the pregnancy and the safe delivery of the baby. Smoking causes premature rupture of the membranes (breaking of the amniotic sac before the onset of labour), placenta previa (when the placenta is attached to the uterine wall close to or over the cervix), and placental abruption (premature separation of the placenta from the wall of the uterus).2, 3, 9, 10 These complications increase the risk of preterm delivery, haemorrhaging that requires a blood transfusion, and death of the mother or baby. Research indicates that stopping smoking between pregnancies reduces the risk of placental abruption, suggesting that, for this complication, the effects of smoking do not persist.3
3.7.4 Preterm delivery
Smoking is a cause of preterm delivery (birth at less than 37 completed weeks of gestation) and shortened gestation.2 Smoking during pregnancy is also associated with an increased risk of extremely preterm birth (<28 weeks of gestation).11 Preterm delivery is a leading cause of neonatal death and illness.3 Australian data show that in 2003, babies born to women who smoked in pregnancy had a 60% higher risk of preterm delivery than babies of non-smoking women.12 It is not known how smoking contributes to preterm delivery, but researchers have proposed various mechanisms. Smokers are more susceptible to vaginal infection; for example they have two to three times the risk of bacterial vaginosis, which is a risk factor for preterm delivery. Some research suggests that smokers may be more sensitive to stimuli that lead to contractions. Smoking may affect collagen formation, leading to weakening and rupture of the membranes. Smokers are more likely to develop complications that are risk factors for preterm delivery, such as placental abruption and placenta previa.3 A recent review concluded that there are multiple mechanisms by which smoking may cause preterm birth: these include nicotine-induced vasoconstriction (constricted blood vessels), carbon monoxide-induced foetal hypoxia (when the foetus is deprived of adequate oxygen), cadmium disruption of calcium signalling, altered steroid hormone production, disruption of prostaglandin synthesis, and changed responses to oxytocin.13 Australian research has also found that maternal smoking and preterm birth interact to further increase a woman’s risk of cardiovascular disease, beyond either risk factor independently.14
Research suggests that women who quit smoking within the first three months of pregnancy reduce their risk of premature birth,15-18 placental complications at birth, infant illness and perinatal death.15
3.7.5 Other health effects
In 2010, one in five Australian mothers of children under two had been diagnosed with depression,19 and there is a well-documented association between smoking and depression (see Section 7.12). A recent meta-analysis concluded that women who smoke during pregnancy are more than twice as likely to develop postnatal depression compared with women who do not smoke.20
Australian research found that, compared with women who did not smoke, women who smoked were more likely to have a caesarean section for non-reassuring foetal status (when test results indicate the foetus is not getting enough oxygen).21
In Australia in 2012–14, maternal death was more common in those who smoked in the first 20 weeks of pregnancy than in those who did not smoke (however, as the number of maternal deaths with a recorded smoking status was small, caution should be taken when interpreting these data).22
Smoking is associated with a lower risk of pre-eclampsia; however, this does not outweigh the risks of prenatal smoking2 —see Section 3.28.
Relevant news and research
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References for Section 3.7
1. Australian Institute of Health and Welfare. Australia’s mothers and babies 2017—in brief. Perinatal statistics series no. 35. Cat. no. PER 100, Canberra: AIHW, 2019. Available from: https://www.aihw.gov.au/getmedia/2a0c22a2-ba27-4ba0-ad47-ebbe51854cd6/aihw-per-100-in-brief.pdf.aspx?inline=true.
2. US Department of Health and Human Services. The health consequences of smoking: A report of the Surgeon General. Atlanta, Georgia: US Department of Health and Human Services, Public Health Service, Centers for Disease Control and Prevention, National Center for Chronic Disease Prevention and Health Promotion, Office on Smoking and Health, 2004. Available from: http://www.cdc.gov/tobacco/data_statistics/sgr/index.htm.
3. US Department of Health and Human Services. How tobacco smoke causes disease: The biology and behavioral basis for smoking-attributable disease. A report of the US Surgeon General, Atlanta, Georgia: US Department of Health and Human Services, Centers for Disease Control and Prevention, National Center for Chronic Disease Prevention and Health Promotion, Office on Smoking and Health, 2010. Available from: http://www.surgeongeneral.gov/library/tobaccosmoke/report/index.html.
4. Pintican D, Poienar AA, Strilciuc S, and Mihu D. Effects of maternal smoking on human placental vascularization: A systematic review. Taiwan J Obstet Gynecol, 2019; 58(4):454-9. Available from: https://www.ncbi.nlm.nih.gov/pubmed/31307732
5. US Department of Health and Human Services. The health consequences of involuntary exposure to tobacco smoke: A report of the Surgeon General. Atlanta, Georgia: US Dept. of Health and Human Services, Centers for Disease Control and Prevention, Coordinating Center for Health Promotion, National Center for Chronic Disease Prevention and Health Promotion, Office on Smoking and Health, 2006. Available from: http://www.cdc.gov/tobacco/data_statistics/sgr/sgr_2006/index.htm.
6. US Department of Health and Human Services. The health consequences of smoking - 50 years of progress. Atlanta, GA: US Department of Health and Human Services, Centers for Disease Control and Prevention, National Center for Chronic Disease Prevention and Health Promotion, Office on Smoking and Health, 2014. Available from: http://www.surgeongeneral.gov/library/reports/50-years-of-progress/.
7. Hyland A, Piazza KM, Hovey KM, Ockene JK, Andrews CA, et al. Associations of lifetime active and passive smoking with spontaneous abortion, stillbirth and tubal ectopic pregnancy: A cross-sectional analysis of historical data from the women's health initiative. Tobacco Control, 2015; 24(4):328-35. Available from: https://www.ncbi.nlm.nih.gov/pubmed/24572626
8. Pineles BL, Park E, and Samet JM. Systematic review and meta-analysis of miscarriage and maternal exposure to tobacco smoke during pregnancy. American Journal of Epidemiology, 2014; 179(7):807-23. Available from: https://www.ncbi.nlm.nih.gov/pubmed/24518810
9. Shobeiri F and Jenabi E. Smoking and placenta previa: A meta-analysis. Journal of Maternal-Fetal and Neonatal Medicine, 2017; 30(24):2985-90. Available from: https://www.ncbi.nlm.nih.gov/pubmed/27936997
10.Shobeiri F, Masoumi SZ, and Jenabi E. The association between maternal smoking and placenta abruption: A meta-analysis. Journal of Maternal-Fetal and Neonatal Medicine, 2017; 30(16):1963-7. Available from: https://www.ncbi.nlm.nih.gov/pubmed/27623712
11.Dahlin S, Gunnerbeck A, Wikstrom AK, Cnattingius S, and Edstedt Bonamy AK. Maternal tobacco use and extremely premature birth - a population-based cohort study. BJOG, 2016; 123(12):1938-46. Available from: https://www.ncbi.nlm.nih.gov/pubmed/27411948
12.Laws P, Grayson N, and Sullivan E. Smoking and pregnancy. AIHW cat. no. PER 33.Sydney: Australian Institute of Health and Welfare National Perinatal Statistics Unit, 2006. Available from: http://www.npsu.unsw.edu.au/NPSUweb.nsf/resources/AMB_2004_2008/$file/Smoking+and+pregnancy+for+web.pdf.
13.Ion R and Bernal AL. Smoking and preterm birth. Reprod Sci, 2015; 22(8):918-26. Available from: https://www.ncbi.nlm.nih.gov/pubmed/25394641
14.Ngo AD, Roberts CL, Chen JS, and Figtree G. Interaction of maternal smoking and preterm birth on future risk of maternal cardiovascular disease: A population-based record linkage study. Eur J Prev Cardiol, 2016; 23(6):613-20. Available from: https://www.ncbi.nlm.nih.gov/pubmed/26157020
15.British Medical Association Board of Science and Education and Tobacco Control Resource Centre, Smoking and reproductive life. The impact of smoking on sexual, reproductive and child health. London: British Medical Association; 2004. Available from: http://www.bma.org.uk/images/smoking_tcm41-21289.pdf.
16.Kondracki AJ and Hofferth SL. A gestational vulnerability window for smoking exposure and the increased risk of preterm birth: How timing and intensity of maternal smoking matter. Reprod Health, 2019; 16(1):43. Available from: https://www.ncbi.nlm.nih.gov/pubmed/30992027
17.Moore E, Blatt K, Chen A, Van Hook J, and DeFranco EA. Relationship of trimester-specific smoking patterns and risk of preterm birth. American Journal of Obstetrics and Gynecology, 2016; 215(1):109 e1-6. Available from: https://www.ncbi.nlm.nih.gov/pubmed/26827877
18.Soneji S and Beltran-Sanchez H. Association of maternal cigarette smoking and smoking cessation with preterm birth. JAMA Netw Open, 2019; 2(4):e192514. Available from: https://www.ncbi.nlm.nih.gov/pubmed/31002320
19.Australian Institute of Health and Welfare. Experience of perinatal depression: Data from the 2010 Australian national infant feeding survey. Information Paper. Cat. no. PHE 161, Canberra: AIHW, 2012. Available from: https://www.aihw.gov.au/reports/primary-health-care/perinatal-depression-data-from-the-2010-australia/contents/summary.
20.Chen HL, Cai JY, Zha ML, and Shen WQ. Prenatal smoking and postpartum depression: A meta-analysis. Journal of Psychosomatic Obstetrics and Gynaecology, 2019; 40(2):97-105. Available from: https://www.ncbi.nlm.nih.gov/pubmed/29514549
21.Li R, Lodge J, Flatley C, and Kumar S. The burden of adverse obstetric and perinatal outcomes from maternal smoking in an Australian cohort. Australian and New Zealand Journal of Obstetrics and Gynaecology, 2019; 59(3):356-61. Available from: https://www.ncbi.nlm.nih.gov/pubmed/30014485
22.Australian Institute of Health and Welfare. Maternal deaths in Australia 2012–2014. Cat. no. PER 92. Canberra: AIHW, 2017. Available from: https://www.aihw.gov.au/reports/mothers-babies/maternal-deaths-in-australia-2012-2014/contents/table-of-contents.