Abstract
We investigated the effect of maternal smoking during pregnancy on the relative risk of sudden infant death syndrome (SIDS) by linking data from Georgia birth and death certificates from 1997 to 2000. We estimated the effect of misclassifying smokers as non-smokers and the effect of being misclassified on SIDS rates, and we calculated the fraction of cases caused by exposure. Of all SIDS cases, 21% were attributable to maternal smoking; among smokers, 61% of SIDS cases were attributable to maternal smoking. Maternal smoking during pregnancy is associated with a significantly increased risk of SIDS.
Sudden infant death syndrome (SIDS) is the sudden death of an infant aged younger than 1 year that remains unexplained after a thorough case investigation that includes an autopsy, a death scene investigation, and a review of the clinical history of the parents and the infant.1
Known risk factors for SIDS include sleeping in the prone position, being exposed to smoke pre- and postnatally, sharing a bed with a mother who smokes, hyperthermia, lack of breastfeeding, and sleeping on soft surfaces.2
Even though the rate of SIDS cases in the United States decreased by 40% from 1992 to 1999, the surgeon general reports that smoking rates during pregnancy may be as high as 22%.3 As shown by Guntheroth in low-income women,4 prenatal exposure to smoking likely means exposure to smoking during pregnancy and after pregnancy as well. According to that study, of the low-income women who smoked, most continued to smoke throughout their pregnancy; of those who quit, most returned to smoking during the pregnancy or shortly after delivery.
Studies have also shown that, on birth certificates, mother’s smoking status has been found to be substantially misclassified.5 Dietz et al.5 used a 2-sample capture–recapture method to estimate the completeness of recorded prenatal smoking on birth certificates in Georgia and found that whereas the reported maternal smoking from 1993 to 1995 was 13.3%, their estimate by the capture–recapture method was 20.8%. Because underreporting and misclassification of smoking status leads to a biased estimate of the smoking–SIDS relationship, we linked birth and infant mortality records from the state of Georgia to examine the effect of maternal smoking during pregnancy on the frequency of SIDS cases in Georgia.
METHODS
We linked birth and death certificate data obtained from the Georgia State Division of Public Health for the January 1, 1997, to December 31, 2000, birth cohort. Using reported smoking as the primary exposure and SIDS as the outcome, we investigated many variables to determine which ones were significant effect modifiers or confounders (maternal smoking, maternal education, maternal race, maternal age, maternal weight gain during pregnancy, alcohol use during pregnancy, plurality, total number of prenatal care visits, mother’s number of previous fetal deaths, mother’s number of previous live births now dead, parity, father’s name present on birth certificate, father’ education, infant’s sex, prematurity, infant birthweight, Apgar scores at 1 and 5). We used stratified analysis followed by logistic regression to evaluate, one at a time, each factor’s possible effect on the association between smoking and SIDS.
We then attempted to estimate a more valid measure of association between maternal smoking and SIDS by correcting the figures we obtained for smoker misclassification on the basis of estimates from Dietz et al.5
RESULTS
The total number of births and deaths during the study period were 510209 and 4495, respectively. Analyses were based on 489494 birth records in which maternal smoking information was available; 81736 (9.0%) mothers reported smoking. A total of 438 SIDS cases were identified during the study period for a rate of 0.9 per 1000 live births. The rate of SIDS in infants born to mothers reported to have smoked during pregnancy was 2.4 per 1000, and the rate was 0.8 per 1000 for non-smokers (Table 1 ▶). The crude risk ratio and crude odds ratio were 3.1 (Table 2 ▶).
TABLE 1—
Distribution of Population, Reported Smoking, and Incidence of Sudden Infant Death Syndrome (SIDS), by Various Factors: Georgia Birth and Death Certificates, 1997–2000
| All Births | Population Distribution, % | Reported Maternal Smoking, % (95% CI) | Incidence of SIDS per 1000 Births (95% CI) |
| Overall population | 100 | 9.0 (8.9, 9.1) | 0.9 (0.8, 1.0) |
| SIDS | |||
| Yes | 0.1 | 23.7 (19.8, 27.7) | |
| No | 99.9 | 9.0 (8.9, 9.1) | |
| Maternal smoking during pregnancy | |||
| Yes | 9.0 | 2.4 (1.9, 2.8) | |
| No | 91.0 | 0.8 (0.7, 0.9) | |
| Mother’s age, y | |||
| < 15 | 0.4 | 3.7 (2.8, 4.6) | 2.3 (0.0, 5.5) |
| 15–25 | 46.4 | 11.1 (11.0, 11.2) | 1.3 (1.2, 1.5) |
| > 25 | 53.2 | 7.3 (7.2, 7.4) | 0.6 (0.5, 0.7) |
| Maternal alcohol use during pregnancy | |||
| Yes | 0.8 | 46.7 (45.1, 48.4) | 1.9 (0.5, 3.3) |
| No | 99.2 | 8.8 (8.7, 8.8) | 0.9 (0.8, 1.0) |
| Mother’s race | |||
| White | 64.0 | 11.7 (11.6, 11.8) | 0.7 (0.6, 0.8) |
| Black | 33.3 | 4.7 (4.6, 4.8) | 1.3 (1.2, 1.5) |
| Other | 2.7 | 2.4 (2.1, 2.6) | 0.6 (0.2, 1.0) |
| Mother’s level of education, y | |||
| < 12 | 23.1 | 17.6 (17.4, 17.8) | 1.7 (1.4, 1.9) |
| 12 | 33.1 | 10.6 (10.4, 10.7) | 0.9 (0.8, 1.1) |
| > 12 | 43.8 | 3.5 (3.4, 3.6) | 0.5 (0.4, 0.6) |
| Mother’s marital status | |||
| Yes | 63.7 | 7.0 (6.9, 7.1) | 0.5 (0.5, 0.6) |
| No | 36.3 | 12.6 (12.5, 12.8) | 1.5 (1.4, 1.7) |
| Child’s gender | |||
| Male | 51.0 | 9.0 (8.9, 9.2) | 1.1 (0.9, 1.2) |
| Female | 49.0 | 9.0 (8.9, 9.2) | 0.8 (0.7, 0.9) |
| Preterm birth | |||
| Yes | 11.9 | 10.8 (10.6, 11.1) | 1.8 (1.5, 2.2) |
| No | 88.1 | 8.8 (8.7, 8.9) | 0.8 (0.7, 0.9) |
| Low birthweight | |||
| Yes | 8.7 | 13.8 (13.4, 14.1) | 2.3 (1.8, 2.7) |
| No | 91.3 | 8.6 (8.5, 8.7) | 0.8 (0.7, 0.9) |
| Mother’s previous fetal deaths | |||
| 0 | 82.1 | 8.3 (8.2, 8.4) | 0.09 (0.08, 0.09) |
| 1 | 12.8 | 11.8 (11.6, 12.1) | 0.11 (0.08, 0.13) |
| 2 | 3.2 | 13.8 (13.2, 14.3) | 0.16 (0.10, 0.22) |
| > 2 | 1.3 | 17.3 (16.3, 18.1) | 0.13 (0.04, 0.21) |
| Mother’s previous pregnancies | |||
| 0 | 42.0 | 6.9 (6.8, 7.1) | 0.06 (0.05, 0.07) |
| 1 | 32.9 | 9.0 (8.9, 9.2) | 0.08 (0.07, 0.10) |
| 2 | 16.0 | 11.3 (11.1, 11.6) | 0.13 (0.11, 0.16) |
| > 2 | 9.1 | 14.5 (14.2, 14.9) | 0.17 (0.14, 0.21) |
Note. CI = confidence interval.
TABLE 2—
Estimated Association of Smoking With Sudden Infant Death Syndrome: Georgia Birth and Death Certificates, 1997–2000
| OR (95% CI) | |
| Crude OR on the basis of reported smoking | 3.1 (2.5, 3.9) |
| Adjusted OR on the basis of reported smokinga | 2.3 (1.8, 2.9) |
| Adjusted OR corrected for misclassification of smokingb | 2.6 (2.1, 3.2) |
| Etiological fraction in population on the basis of adjusted andcorrected OR | 20.7% (15.4, 26.1) |
| Etiological fraction in smokers on the basis of adjusted and corrected OR | 61.3% (52.8, 68.3) |
Note. OR = odds ratio; CI = confidence interval.
aAdjusted for gestational age, low birthweight, parity, maternal education, infant gender, and maternal marital status.
bCorrected for smoking misclassification, assuming a sensitivity of 54.5% and a specificity of 99.9%.
Table 1 ▶ shows the distribution of population, reported smoking, and incidence of SIDS, by the various factors that significantly affected the smoking–SIDS relation. Certain factors such as weight gain, prenatal care, Apgar scores, and paternal factors did not significantly affect the smoking–SIDS relation (data not presented).
The prevalence of reported smoking on birth certificates was 9.0%; when we used the capture–recapture method as described by Dietz et al.,5 we estimated the proportion of women who smoked during pregnancy to be 16.5%. When we applied this rate to the adjusted odds ratio of 2.3, we estimated the true odds ratio for the smoking–SIDS association (after we controlled for the previously mentioned factors and accounted for misclassification) to be 2.6, with an etiological fraction (the fraction of cases caused by exposure) in the population of 20.7% and etiological fraction in smokers of 61.3% (Table 2 ▶).
DISCUSSION
Smoking was found to be an important risk factor for SIDS, with an estimated odds ratio of 2.6 after we controlled for confounders and adjusted for smoking status misclassification on birth certificates. We estimated that 20.7% of SIDS cases could have been prevented if women had not smoked during pregnancy (etiological fraction in the population). We also estimated that 61.3% of the SIDS cases in children born to women who smoked during pregnancy were a result of smoking (i.e., etiological fraction in the exposed).
This study was based on birth and death certificate data. The information on maternal smoking and other maternal behaviors was self-reported, which may have led to maternal smoking misclassification. It was not possible to assess the effect of sleeping position on SIDS in this study.
Given the current level of understanding of the mechanisms by which SIDS occurs, public health programs have concentrated on avoidance of modifiable risk factors. Maternal smoking during pregnancy appears to be the primary modifiable risk factor for SIDS.6–15
Acknowledgments
The authors thank the state of Georgia, Division of Public Health, for making the data available and for providing other information on the reporting of births and deaths in the state.
Human Participant Protection No protocol or institutional review board approval was needed for this study because the data received from the Division of Public Health were devoid of any unique identifiers.
Peer Reviewed
Contributors T. Shah and K. Sullivan designed the study, performed the analysis, and wrote the brief. J. Carter was instrumental in procuring the data for the study.
References
- 1.Willinger M, James LS, Catz C. Defining the sudden infant death syndrome (SIDS): deliberations of an expert panel convened by the National Institute of Child Health and Human Development. Pediatr Pathol. 1991;11:677–684. [DOI] [PubMed] [Google Scholar]
- 2.Georgia Division of Public Health. Sudden Infant Death Syndrome (SIDS) and Other Infant Death (OID) Information, Support and Referral Program. Available at: http://health.state.ga.us/programs/sids/index.asp. Accessed January 20, 2005.
- 3.Guntheroth WG. Crib Death: The Sudden Infant Death Syndrome. 3rd ed. Seattle, Wash: University of Washington, Futura Publishing Co; 1995.
- 4.US Dept of Health and Human Services. Women and Smoking: A Report of the Surgeon General. Washington, DC: Office of the Surgeon General; 2001.
- 5.Dietz PM, Adams MM, Kendrick JS, Mathis MP, PRAMS Working Group. Completeness of ascertainment of prenatal smoking using birth certificates and confidential questionnaires. Am J Epidemiol. 1998;148: 1048–1054. [DOI] [PubMed] [Google Scholar]
- 6.Alm B, Milerad J, Wennergren G. A case-control study of smoking and sudden infant death syndrome in the Scandinavian countries, 1992 to 1995. The Nordic Epidemiological SIDS Study. Arch Dis Child. 1998;78: 329–334. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 7.Alm B, Wennergren G, Norvenius G. Caffeine and alcohol as risk factors for sudden infant death syndrome. Nordic Epidemiological SIDS Study. Arch Dis Child. 1999;81:107–111. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 8.Daltveit AK, Irgens LM, Oyen N, Skjaerven R, Markestad T, Wennergren G. Circadian variations in sudden infant death syndrome: associations with maternal smoking, sleeping position and infections. The Nordic Epidemiological SIDS Study. Acta Paediatr. 2003;92:1007–1013. [PubMed] [Google Scholar]
- 9.Schoendorf KC, Parker JD, Batkhan LZ, Kiely JL. Comparability of the birth certificate and 1988 Maternal and Infant Health Survey. Vital Health Stat 2. 1993; No. 116:1–19. [PubMed]
- 10.Ruggiero L, Bane CM, Dryfoos JM, Rossi JS. Development of a pregnancy-tailored decisional balance measure for smoking cessation. Addict Behav. 1999;24:795–799. [DOI] [PubMed] [Google Scholar]
- 11.Taylor JA, Sanderson M. A reexamination of the risk-factors for the sudden infant death syndrome. J Pediatr. 1995;126:887–891. [DOI] [PubMed] [Google Scholar]
- 12.Klonoff-Cohen HS, Edelstein SL, Lefkowitz ES. The effect of passive smoking and tobacco exposure through breast milk on sudden infant death syndrome. JAMA. 1995;273:795–798. [DOI] [PubMed] [Google Scholar]
- 13.Naeye RL, Ladis B, Drage JS. Sudden infant death syndrome: a prospective study. Am J Dis Child. 1976; 130:1207–1210. [DOI] [PubMed] [Google Scholar]
- 14.Valdes-Dapena M. Sudden infant death syndrome: overview of recent research developments from a pathologist’s perspective. Pediatrician. 1988;15:222–230. [PubMed] [Google Scholar]
- 15.Malloy MH, Hoffman HJ, Peterson DR. Sudden infant death syndrome and maternal smoking. Am J Public Health. 1992;82:1380–1382. [DOI] [PMC free article] [PubMed] [Google Scholar]