Smoking and the sudden infant death syndrome

The aims of this review are (a) to critically examine the epidemiologic evidence for a possible association between smoking and the sudden infant death syndrome (SIDS), (b) to review the pathology and postulated physiological mechanism(s) by which smoking might be causally related to SIDS, and (c) to provide recommendations for SIDS prevention in relation to tobacco smoking. Over 60 studies have examined the relation between maternal smoking during pregnancy and risk of SIDS. With regard to prone-sleep-position intervention programs, the pooled relative risk associated with maternal smoking was RR = 2.86 (95% CI = 2.77, 2.95) before and RR = 3.93 (95% CI = 3.78, 4.08) after. Epidemiologically, to distinguish the effect of active maternal smoking during pregnancy from involuntary tobacco smoking by the infants of smoking mothers is difficult. Clear evidence for environmental tobacco smoke exposure can be obtained by examining the risk of SIDS from paternal smoking when the mother is a non-smoker. Seven such studies have been carried out. The pooled unadjusted RR was 1.49 (95% CI = 1.25, 1.77). Consideration of the pathological and physiological effects of tobacco suggests that the predominant effect from maternal smoking comes from the in utero exposure of the fetus to tobacco smoke. Assuming a causal association between smoking and SIDS, about one-third of SIDS deaths might have been prevented if all fetuses had not been exposed to maternal smoking in utero.     

Maternal pre-eclampsia/eclampsia and the risk of sudden infant death syndrome in offspring

To determine whether maternal exposure to pre-eclampsia/eclampsia during pregnancy increases the risk of sudden infant death syndrome (SIDS) in offspring, we conducted a population-based case-control study using the California linked birth and death certificate data. All infants who died of SIDS (ICD-9 code 798.0) during 1989-91 were identified as cases. More than 96% of the identified SIDS cases were diagnosed through autopsy. Ten controls who did not die from SIDS were randomly selected for each case from the birth certificate matched to the case on the year of birth. Among 2,029 cases and 21,037 controls included in the final analysis, mothers of 49 cases (2.4%) and 406 controls (1.9%) had a diagnosis of either pre-eclampsia or eclampsia noted on the birth certificate. After adjustment for maternal age, prenatal smoking, race/ethnicity, parity, maternal education, gestational age at the initial visit for prenatal care, infant year of birth and infant sex, maternal pre-eclampsia/ eclampsia during pregnancy was associated with a 50% increased risk of SIDS in the offspring (odds ratio = 1.5, 95% confidence interval 1.1, 2.0). Potential under-reporting of pre-eclampsia/eclampsia on the birth certificates was likely to be non-differential and is unlikely to explain the finding. Fetal hypoxia resulting from pre-eclampsia/ eclampsia or immunological aetiology affecting the risk of both pre-eclampsia/eclampsia and SIDS may explain the finding.     

Sudden infant death syndrome, maternal smoking during pregnancy, and the cost-effectiveness of smoking cessation intervention

OBJECTIVES: This study analyzed the relationship between prenatal maternal smoking and sudden infant death syndrome (SIDS) and examined the cost-effectiveness of smoking cessation interventions. METHODS: All recorded US singleton SIDS deaths from the 1995 birth cohort with birthweight exceeding 500 g were investigated. Infants with available maternal smoking data were matched with controls who survived to 1 year. Conditional logistic regression was used to estimate SIDS risks and accompanying cost-effectiveness. RESULTS: A total of 23.6% of singleton SIDS deaths appear to be attributable to prenatal maternal smoking. Typical cessation services available to all pregnant smokers could avert 108 SIDS deaths annually, at an estimated cost of $210,500 per life saved. CONCLUSIONS: Typical prenatal smoking cessation programs are highly cost-effective but have limited impact on the population incidence of SIDS.     

Maternal smoking and low birthweight: implications for antenatal care.

The incidence of low birthweight has been related to smoking prevalence in each social group using published data for 1984. The attributable risk of low birthweight has been estimated, based on a relative risk of 2 for mothers who smoke during pregnancy. Assuming 12.5% of cigarette smokers stopped smoking during pregnancy, 18.1% of all low weight births were caused by maternal smoking in 1984. The percentage for most social groups was similar. The overall attributable risk from smoking was estimated to be 12.7 low weight births per 1000 total births, with a further 12.1 per 1000 due to other factors acting in a socioeconomic gradient. We estimate that the minimum attainable low birthweight incidence in 1984 was 45.4 per 1000 total births, based on the lowest observed incidence, corrected for smoking prevalence, which was in social group II. We recommend the addition of maternal smoking information to the Korner maternity clinical options data set, to enable an accurate assessment of the risks and to provide local monitoring of initiatives to reduce smoking prevalence during pregnancy.     

Prenatal and intrapartum events and sudden infant death syndrome

The purpose of this study was to evaluate specific pregnancy and labour and delivery events that may increase the risk of sudden infant death syndrome (SIDS). A matched case-control study was conducted in five counties in southern California, using California death certificate records. The sample consisted of 239 Caucasian, African-American, Hispanic and Asian mothers of SIDS infants and 239 mothers of control infants matched on sex, race, birth hospital and date of birth. Mothers participated in a detailed telephone interview and provided access to obstetric and paediatric records. More case than control mothers reported a family history of anaemia (OR=2.12, P < 0.001). Placental abruptions were strongly associated with SIDS (unadjusted OR=7.94, [95% CI 1.34,47.12]). There was an increased risk of SIDS death associated with maternal anaemia during pregnancy (OR=2.51, [95% CI 1.25,5.03]), while simultaneously adjusting for maternal smoking during pregnancy, maternal years of education and age, parity, infant birthweight, gestational age, medical conditions at birth, infant sleep position and post-natal smoking. Interactions of anaemia and prenatal smoking as well as anaemia and post-natal smoking were not statistically significant. There were no other statistically significant differences between case and control mothers for pregnancy conditions, labour and delivery events (e.g. caesarean sections, anaesthesia, forceps) or newborn complications (e.g. nuchal cord, meconium aspiration). Anaemia and placental abruptions were significantly associated with an increased risk of SIDS; both are circumstances in which a fetus may become hypoxic, thereby compromising the subsequent growth, development and ultimate survival of the infant.     

Levels of Excess Infant Deaths Attributable to Maternal Smoking During Pregnancy in the United States

Objectives: The objectives of the study were: 1) To determine the risk of infant mortality associated with prenatal cigarette smoking; 2) To assess whether the relationship, if existent, was dose-dependent; 3) To explore the morbidity pathway that explains the effect of tobacco smoke on infant mortality, and 4) to compute excess infant deaths attributable to maternal smoking in the United States. Methods: Retrospective cohort study on 3,004,616 singleton live births that occurred in 1997 in the United States using the US national linked birth/infant death data. Excess infant deaths due to maternal smoking were computed using the population-attributable risk (PAR). Results: Overall, 13.2% of pregnant women who delivered live births in 1997 smoked during pregnancy. The rate of infant mortality was 40% higher in this group as compared to nonsmoking gravidas (P < 0.0001). This risk increased with the amount of cigarettes consumed prenatally in a dose-dependent fashion (p for trend <0.0001). Small-for-gestational age rather than preterm birth is the main mechanism through which smoking causes excess infant mortality. We estimated that about 5% of infant deaths in the United States were attributable to maternal smoking while pregnant, with variations by race/ethnicity. The proportion of infant deaths attributable to maternal smoking was highest among American Indians at 13%, almost three times the national average. If pregnant smokers were to halt tobacco use a total of 986 infant deaths would be averted annually. Conclusions: Smoking during pregnancy accounts for a sizeable number of infant deaths in the United States. This highlights the need for infusion of more resources into existing smoking cessation campaigns in order to achieve higher quit rates, and substantially diminish current levels of smoking-associated infant deaths.     

Cigarette smoking as a risk factor for sudden infant death syndrome: a population-based study.

Risk factors for sudden infant death syndrome (SIDS) were examined in a prospective study based on Swedish births between 1983 and 1985. All infants surviving the first week of life were included (279,938). The overall rate of SIDS was 0.7 per 1,000 first week survivors. Elevated relative risks were associated with low maternal age, multiparity, maternal smoking, and male infants. Smoking doubled the risk and a clear dose-response relation by amount smoked was observed. Maternal smoking also seemed to influence the time of death, as infants of smokers died at an earlier age. In countries like Sweden, smoking may be the single most important preventable risk factor for sudden infant death syndrome.     

Maternal smoking, birthweight and gestational age in sudden infant death syndrome (SIDS) babies and their surviving siblings

To evaluate the effect of maternal smoking on intrauterine growth of babies who died of sudden infant death syndrome (SIDS), birthweights of SIDS infants and their surviving siblings were compared with birthweights of infants in sibships were all infants survived the first year of life. We studied 184 349 mothers with at least two births registered in the population-based Swedish Medical Birth Registry during 1983–91. The mother being the unit of analysis, birthweight and gestational age of her infants were the repeated measures used in a repeated measures analysis of variance. Mothers whose first two infants survived at least 1 year, smoked less than mothers of SIDS infants, 25 and 41% ( P < 5 0.01). Overall, SIDS mothers did not smoke more while pregnant with the SIDS infant than while pregnant with the surviving sibling. SIDS siblings weighted, on average, 90 g less than infants in non-affected sibships. SIDS babies were even lighter, 193 g, and had 3.8 days shorter mean gestational age, compared with same birth-order babies in non-affected sibships. After adjustment for gestational age, the birthweight difference changed only slightly for SIDS siblings, while the difference for SIDS infants was reduced from 193 to 110 g. Further adjustment for smoking reduced the birthweight difference for SIDS siblings, from 74 to 50 g, and SIDS infants, from 110 to 82 g. Intrauterine growth retardation of sibships with a SIDS baby is explained only partly by maternal smoking. The even lower birthweight of the SIDS baby, resulting from shorter gestational age, cannot be explained by smoking, suggesting pregnancy factors specific to the SIDS baby and not to its siblings.     

Cigarette smoking during pregnancy in rural Nepal. Risk factors and effects of beta-carotene and vitamin A supplementation

OBJECTIVE: We examined risk factors of smoking and the association between smoking and pregnancy-related and 6-month infant mortality in rural Nepal, where 30% women reported smoking during pregnancy. DESIGN: Cross-sectional analysis of risk factors associated with smoking status and health consequences of smoking, using prospective data collected as part of a randomized community trial to examine the effect of maternal vitamin A or beta-carotene supplementation on maternal mortality. SETTING: Rural, southeastern plains of Nepal. SUBJECTS AND METHODS: A total of 17 767 women contributed at least one pregnancy during 3.5 y of the study. Data on cigarette or bidi (rolled tobacco) smoking were collected using a 7-day recall, twice during pregnancy. Associations between smoking status and maternal diet, morbidity profile, household socioeconomic status and serum concentration of retinol, carotenoids and tocopherols were examined. Further, relative risk (RR) and 95% confidence intervals (CI) were calculated to estimate supplement effects on pregnancy-related mortality, stratified by smoking status during pregnancy. RESULTS: Smokers were more likely to be older, illiterate and poor compared to nonsmokers. Fruit and vegetable consumption among smokers and nonsmokers did not vary. However, smokers were more likely to consume meat/fish/eggs and less likely to consume milk than nonsmokers. They were also more likely to report symptoms of vaginal bleeding, edema, severe headache and convulsions during pregnancy relative to nonsmokers. Mortality per 100,000 pregnancies appeared to be higher among smokers than nonsmokers in the placebo group (915 vs 584, RR=1.57, 95% CI: 0.80-3.08). beta-Carotene supplementation reduced pregnancy-related mortality both among smokers (RR=0.31 95% CI: 0.11-0.89) and nonsmokers (RR=0.41, 95% CI: 0.19-0.89). Similar results obtained with vitamin A supplementation were not statistically significant. Infant mortality up to 6 months was approximately 30% higher among smokers compared to nonsmokers in the placebo group both before and after adjusting for confounding factors. Neither supplement given to women reduced infant mortality. CONCLUSIONS: Cigarette smoking during pregnancy is associated with an increased risk of maternal and infant mortality in rural Nepal. beta-Carotene and to some extent vitamin A may reduce the risk of pregnancy-related mortality, but not infant mortality, among both smokers and nonsmokers.     

Sudden infant death syndrome in New Zealand: are risk scores useful? New Zealand National Cot Death Study Group.

STUDY OBJECTIVE--To evaluate the Christchurch, Invercargill, Dunedin (CID) and Oxford record linkage study (ORLS) risk scores in five regions of New Zealand and examine the effect of risk factors for sudden infant death syndrome (SIDS), such as prone sleeping position, maternal smoking, breast feeding, measures of illness, the use of antenatal classes, community health care, and medical services on a high and low risk group delineated by the CID score. DESIGN--This was a case-control study of infants dying of SIDS. SETTING--Both the cases and controls were born in one of five health districts in New Zealand and their parents were interviewed between 1 November 1987 and 31 October 1990. PARTICIPANTS--The cases were 485 infants who died of SIDS. The controls were a random sample drawn from the same five regions in which the cases were born, chosen so that their age on the day on which they were interviewed was similar to the age at death of the cases. Risk scores were calculated for 387 case and 1579 controls. MEASUREMENTS AND MAIN RESULTS--Using the recommended cut off points the sensitivity and specificity of the CID and ORLS were found to be similar to those described for other samples. The differences among the regions were significant. There was, however, no evidence that the association between SIDS and the risk factors considered was different in the high and low risk groups delineated by the CID score. The relative attributable risk for smoking was 32.3% in the high risk group. The excess risk that could be attributed to a different prevalence of any of the other risk factors in the high risk group was small when compared with the low risk group. CONCLUSIONS--Health care resources should be spent on promoting and evaluating good child care practices for all, rather than identifying and promoting special interventions for those in the high risk category.     

The impact of maternal smoking during pregnancy on delivery outcome

BACKGROUND: Maternal smoking during pregnancy is associated with pre-term birth, intrauterine growth retardation, a small head circumference, a low Apgar score at 5 min and stillbirths and neonatal deaths. This study was undertaken in order to investigate the impact of maternal smoking during pregnancy when all these outcomes were considered. METHODS: With the use of the Swedish Medical Birth Registry, infants in any one of the above mentioned outcome groups were selected from 1,413,811 infants born between 1983 and 1996 with known smoking exposure in early pregnancy. Confounders such as year of birth, maternal age, parity and educational level were controlled for. The attributable risk of maternal smoking on the various negative delivery outcomes was obtained by application of the risk estimates to population counts. RESULTS: The present study confirmed the associations between maternal smoking and the miscellaneous outcomes mentioned above with high significance. The odds ratios (with 95% confidence intervals) for maternal smoking (< 10 cigarettes/day and > or = 10 cigarettes/day) for any one of the outcomes were 1.39 (1.37-1.41) and 1.65 (1.62-1.68) respectively (dose-response p < 0.001). The number of attributable cases caused by maternal smoking was estimated at 15,000, which represents 9% of all cases and 1% of all infants born in Sweden during the study period. CONCLUSION: Maternal smoking during pregnancy accounts for a substantial part of various negative delivery outcomes.     

Risk factors for sudden infant death syndrome in the US Collaborative Perinatal Project

Risk factors for sudden infant death syndrome (SIDS) were studied among infants born to the nearly 56,000 women enrolled in the US Collaborative Perinatal Project from 1959 through 1966. The 193 SIDS cases identified in the cohort were compared with 1930 controls randomly selected from infants who survived the first year of life. The previously documented excess risk associated with black race disappeared after adjusting for maternal education and family income. Maternal smoking, maternal anaemia during pregnancy, and lack of early prenatal care were all positively associated with SIDS. After adjustment for gestational age, infants with low weight and length at birth were still at increased SIDS risk, suggesting that intrauterine growth retardation may be a risk factor. Neurological abnormalities diagnosed before death were associated with SIDS, but much of the association was removed by adjusting for birthweight. The negative association of breastfeeding with SIDS was much reduced upon adjustment by maternal education and birthweight. These findings may have important implications in our understanding of the epidemiology of SIDS.     

Maternal smoking during pregnancy and expenditures on neonatal health care

Cigarette smoking during pregnancy substantially increases the risk of low-weight birth, and infants born to smokers weigh less, on average, than those born to nonsmokers. Low birth weight is an important predictor of neonatal morbidity and death, and the intensity of neonatal care is significantly higher for low-birth-weight infants. In this study, we estimate expenditures on neonatal care based on the relation between maternal smoking during pregnancy and the incidence of low-weight births. Our results indicate that maternal smoking during pregnancy was responsible for 35,816 low-weight births in the U.S. in 1983, or about 14.5% of all low-weight births. We also estimate that 14,977, or 6.6%, of all admissions to neonatal intensive care units were a result of maternal smoking and that these admissions cost approximately $272 million, representing 8.5% of total national expenditures on neonatal intensive care services. We estimate that the average cost of neonatal care was $288 higher for infants born to smokers than for those born to nonsmokers.     

The effect of cigarette smoking on the risk of preeclampsia and gestational hypertension

This case-control study assessed the relation of cigarette smoking during pregnancy to the risk of preeclampsia and gestational hypertension. All subjects were primiparous women without a history of high blood pressure who gave birth in Quebec City or Montreal, Canada, hospitals between 1984 and 1986. Cases (172 women with preeclampsia and 251 with gestational hypertension) and 505 controls were interviewed at the hospital after delivery. Adjusted relative risks were estimated by polychotomous logistic regression. Compared with women who had never smoked, women who were smokers at the onset of pregnancy had a reduced risk of preeclampsia (relative risk = 0.51, 95% confidence interval 0.34-0.77). Relative risks of preeclampsia decreased with increases in the number of cigarettes smoked daily at the onset of pregnancy: Relative risks among smokers of less than 11, 11-20, and more than 20 cigarettes per day were 0.79, 0.56, and 0.38, respectively (test for trend: p = 0.0002). The protective effect of smoking on preeclampsia was stronger for women who continued to smoke after 20 weeks of pregnancy. While smoking tended to reduce the risk of gestational hypertension, this effect was less evident than that for preeclampsia. Relative risks varied little with severity of disease as based on gestational age at the onset of hypertension, maximal blood pressure and, for preeclampsia, amount of proteinuria. The reduction in mean birth weight attributable to smoking during pregnancy was similar among cases and controls. Nicotine inhibition of thromboxane A2 production might explain the decreased risk of pregnancy-induced hypertension among smokers. Despite these findings, the harmful consequences of smoking on pregnancy outcome outweigh its protective effect against pregnancy-induced hypertension.     

Tobacco smoke exposure at one month of age and subsequent risk of SIDS--a prospective study

The aim of this investigation was to identify the sources of postnatal exposure to tobacco smoke at 1 month of age and to examine their relation to sudden infant death syndrome (SIDS). The Tasmanian Infant Health Survey was a prospective cohort study undertaken from 1988 to 1995. It involved 9,826 infants (89% of eligible infants) at higher risk of SIDS. Subsequently 53 eligible infants died of SIDS. Hospital interviews were available on 51 and home interviews on 35 SIDS infants. Urinary cotinine assays were conducted using gas-liquid chromatography (n = 100). Within a predictive model that explained 63% of urinary cotinine variance, the strongest predictor of cotinine and also of SIDS was maternal smoking, though the effects of prenatal and postnatal smoking could not be separated. However, for particular smoking-related behaviors, there was a discordance between prediction of cotinine concentration and prediction of risk of SIDS. If smoking mothers did not smoke in the room with the baby, the cotinine level in the infant's urine was reduced by a little more than a half (p = 0.009), but this was not associated with a reduction in SIDS risk (odds ratio = 1.09, 95% confidence interval 0.47-2.55). Similarly, the presence of other adult resident smokers was associated with a 63% increase in urinary cotinine (p = 0.047) but not with increased SIDS risk (odds ratio = 0.69, 95% confidence interval 0.34-1.40). However, the study lacked the power to detect modest effects, that is, those altering risk less than twofold.     

Smoking in early gestation or through pregnancy: a decision crucial to pregnancy outcome

OBJECTIVE: To assess maternal risk profile and pregnancy outcome of women who continued to smoke, reduced smoking to less than five cigarettes per day or did not smoke during pregnancy. METHODS: We analyzed a population-based database of 26,414 singleton pregnancies from 1989 to 2001. Odds ratios (ORs) for adverse pregnancy outcomes were obtained from multiple logistic regression models. RESULTS: The prevalence of smoking in early pregnancy was 25.7% and went down to 12.7% at 20 weeks of pregnancy. Women who smoked were more often young, primiparous or unmarried, used alcohol and more often had prior pregnancy terminations than women who did not smoke. Women who reduced smoking were mostly primiparous, and they also quit using alcohol. Continuing to smoke was associated with elevated risks of small-for-gestational-age infants (SGA) (OR 2.11), preterm birth (OR 1.15) and perinatal death (OR 1.15). SGA was avoided by reducing smoking, but not prematurity (OR 1.18) or elevated risk of perinatal death (OR 1.18). CONCLUSION: Smoking in early gestation and through pregnancy is associated with adverse pregnancy outcomes, and some of the harmful effects can be avoided by reducing smoking, although the behavioral risk profile of those who reduced smoking is closer to that of smokers than of non-smokers.     

Maternal placental abnormality and the risk of sudden infant death syndrome

To determine whether placental abnormality (placental abruption or placental previa) during pregnancy predisposes an infant to a high risk of sudden infant death syndrome (SIDS), the authors conducted a population-based case-control study using 1989-1991 California linked birth and death certificate data. They identified 2,107 SIDS cases, 96% of whom were diagnosed through autopsy. Ten controls were randomly selected for each case from the same linked birth-death certificate data, matched to the case on year of birth. About 1.4% of mothers of cases and 0.7% of mothers of controls had either placental abruption or placenta previa during the index pregnancy. After adjustment for potential confounders, placental abnormality during pregnancy was associated with a twofold increase in the risk of SIDS in offspring (odds ratio = 2.1, 95% confidence interval 1.3-3.1). The individual effects of placental abruption and placenta previa on the risk of SIDS did not differ significantly. An impaired fetal development due to placental abnormality may predispose an infant to a high risk of SIDS.     

Smoking during pregnancy--United States, 1990-2002

Cigarette smoking during pregnancy adversely affects the health of both mother and child. The risk for adverse maternal conditions (e.g., premature rupture of membranes, abruptio placentae, and placenta previa) and poor pregnancy outcomes (e.g., neonatal mortality and stillbirth, preterm delivery, and sudden infant death syndrome) is increased by maternal smoking. Infants born to mothers who smoke weigh less than other infants, and low birthweight (<2,500 grams) is a key predictor for infant mortality. Infertility and conception delay also might be elevated by smoking. National health objectives for 2010 target an increase in cessation to 30% among pregnant smokers during the first trimester and abstinence from cigarettes by 99% of women giving birth. To assess progress toward these goals, CDC analyzed state-specific trends in maternal smoking during 1990-2002 by using data collected on birth certificates. This report summarizes the results of those analyses, which indicated that whereas participating areas observed a significant decline in maternal smoking during the surveillance period, 10 states reported recent increases in smoking by pregnant teens. Although the widespread public health message to abstain from smoking during pregnancy has helped decrease maternal smoking, to reduce prevalence further, implementation of additional interventions are required.     

Pregnant smokers who quit,pregnant smokers who don't: does history of problem behavior make a difference?

More than half of women who smoke in the USA continue to do so while pregnant. While socioeconomic and demographic factors that distinguish pregnancy quitters from persistent smokers have been identified, less is known about behavioral factors that are associated with persistent smoking. Because smoking during pregnancy is not only an individual, but also a maternal behavior, it may have different behavioral correlates than women's smoking has in general. We propose a conceptual framework in which smoking during pregnancy is viewed as a maternal problem behavior. We explore this conceptualization by examining whether persistent smoking during pregnancy is associated with a pattern of psychosocial risk- and health-compromising behaviors in multiple domains, with pilot data from a small clinic-based sample. Data are presented for 96 predominantly Caucasian, working-class pregnant women recruited from prenatal clinics in the USA. Smoking during pregnancy was measured repeatedly by self-report and biochemical assay. Participants were non-smokers (37%), pregnancy quitters (17%), and persistent smokers (46%). These groups were compared in terms of their history of problem behavior in three domains: interpersonal difficulties, problems in adaptive functioning and problematic health behaviors. With few exceptions, smokers were more likely to have problematic relationships, poorer adaptive functioning and to engage in problematic health behaviors, than both pregnancy quitters and non-smokers. This pattern of problem behavior may interfere with the effectiveness of standard public health prenatal cessation interventions for a sub-group of women. Examining pregnancy smoking as part of a broader matrix of problem behavior may help to identify pregnant women most at risk for persistent smoking and inform the development of targeted interventions.