Sudden infant death syndrome (SIDS) is the sudden death of an infant less than 1 year of age that remains unexplained after a complete postmortem investigation, including autopsy, examination of the death scene, and review of the case history. SIDS, which usually occurs during sleep, is the leading cause of death in the postneonatal period (i.e., from 28 days through 364 days) and the eighth leading cause of years of potential life lost in the United States (1). The risk for SIDS is greatest for infants aged 1-4 months and during the cold season of the year (2); however, an independent contribution of the season of birth to the etiology of SIDS has not been consistently demonstrated (3). This report summarizes an assessment of the association between the risk for SIDS and an infant's month of birth, month of death, and age at death.

Death certificates were analyzed for 112,804 infants aged 1-11 months who died in the United States from 1980 through 1987. The underlying causes of postneonatal death were classified into two major categories: SIDS (International Classification of Diseases, Ninth Revision (ICD-9) rubric 798.0; n=39,379) and all other causes (n=73,425). For each infant, month of birth was derived based on month of death and age at death. The average monthly numbers of live births by race were used as approximate populations at risk. The monthly numbers of deaths were standardized to reflect the different number of days per month. Binomial regression (4) was used to fit the infant's age at death, month of death, and month of birth as categorical variables in a multivariate model predicting the risk for SIDS (5). Relative risks (RRs) were estimated using the first month as reference.

From 1980 through 1987, the risk for SIDS was greater for black infants than white infants and for males than females (Table 1). Among white infants, the risk for SIDS was greatest for those whose mothers resided in the West, and among black infants, for those whose mothers resided in the North Central region. Overall, autopsies were performed for 87.1% of infants who died from SIDS, although the autopsy rate ranged from 82.6% in 1980 to 91.9% in 1987. Findings in this study did not change after exclusion of infants who died from SIDS but were not autopsied.

The RR for SIDS peaked at the second completed month of life (Figure 1). This peak was statistically different from that for infants who died from other causes of postneonatal death (p less than 0.05).

The RR for SIDS was 0.5 times (95% confidence interval (CI)=0.5-0.6) less likely for infants who died in July or August than for those who died in January (Figure 2). The risk for infant deaths from other postneonatal causes was also lower in August than in January (RR=0.7; 95% CI=0.7-0.8), although the magnitude of the effect was different. Postneonatal deaths showed similar January-to-July ratios for SIDS and infectious diseases (2.1 and 2.0, respectively); however, January-to-July ratios were substantially lower for birth defects and external causes (1.3 and 1.0, respectively). The same seasonal pattern held when season of death was examined according to the infant's race, sex, and age at death and according to the region of the United States where the mother resided.

Month of birth was independently associated with SIDS (likelihood ratio test=79.0; degrees of freedom (df)=11; p less than 0.001) after adjusting for age at death and month of death. Month of birth was also independently associated with other causes of postneonatal death (likelihood ratio test=69.8; df=11; p less than 0.001). However, the magnitude of this association was smaller than for month of death. The risk for SIDS deaths for infants born in March (RR=0.9; 95% CI=0.8-0.9) was statistically different from the risk for infants born in September (RR=1.1; 95% CI=1.0-1.1); however, the risk was not statistically different from the risk for other causes of postneonatal death. Reported by: Pregnancy and Infant Health Br, Div of Reproductive Health, Center for Chronic Disease Prevention and Health Promotion, CDC.

Editorial Note

Editorial Note: Since 1975, SIDS has been recognized as a specific clinical diagnosis (6). The high autopsy rate for SIDS cases and the distinct distribution of age at death described in this report strengthen the validity of the underlying cause of death recorded on the death certificate.

The importance of the higher risk for SIDS among white infants in the West is unknown. No other major cause of infant death has been associated with a West-to-East gradient among white infants (7). The higher SIDS rates among black infants in the North Central region probably reflect that region's higher mortality rates for most causes of death among black infants (7).

Several risk factors for SIDS have been established, including infant's of age 1-4 months, male sex, and low birth weight; medical complications of pregnancy and delivery, including multiple births; and the cold season of the year (8,9). However, of these the only risk factor specific for SIDS was infant's age of 1-4 months.

A recent study that addressed the possible etiologic relationship between respiratory infections and SIDS detected an association between diarrhea and/or vomiting during the 2 weeks preceding death and the risk for SIDS (8). The findings of that study also indicated that breastfeeding was protective against SIDS, consistent with an effect mediated through the prevention of gastrointestinal and/or respiratory illnesses (8). Because nonseasonal outbreaks of infectious diseases have not been associated with increased risk for SIDS (2,9), some investigators have suggested that an infectious process during cold weather might precipitate SIDS in developmentally vulnerable infants.

Results of an investigation in England and Wales (5) also suggested a weak association between season of birth and risk for SIDS that was independent of the infant's age at death and season of death. The findings suggested that risk for SIDS and other causes of postneonatal death may be related to the periconceptional or perinatal periods. In the United States, preterm deliveries and perinatal deaths exhibited a seasonal pattern, with a peak in the fall and a trough in the spring (10,11), similar to the pattern documented for the month of birth among SIDS victims described in this report. Explanations for these findings include a possible seasonal association between ascending reproductive tract infections and adverse reproductive outcomes (11).

Although infants at high risk for SIDS cannot be identified early, several maternal, neonatal, and postneonatal factors associated with such increased risk have been identified (8). Parents and health-care providers should be aware of the increased risk for SIDS during the winter season in the United States.

References

1. CDC. Years of potential life lost before ages 65 and

85--United States, 1987 and 1988. MMWR 1990;39:20-2.

2. Peterson DR. Evolution of the epidemiology of sudden infant death syndrome. Epidemiol Rev 1980;2:97-112.

3. Helweg-Larsen K, Bay H, Mac F. A statistical analysis of the seasonality in sudden infant death syndrome. Int J Epidemiol 1985;14:566-74.

4. Wacholder S. Binomial regression in GLIM: estimating risk ratios and risk differences. Am J Epidemiol 1986;123:174-84.

5. Osmond C, Murphy M. Seasonality in the sudden infant death syndrome. Paediatr Perinat Epidemiol 1988;2:337-45.

6. NCHS. Nosology guidelines: cause of death coding manual (supplement). Rockville, Maryland: US Department of Health, Education, and Welfare, Public Health Service, Health Resources and Services Administration, 1975; DHEW publication no. (HRA)75-1140.

7. Allen DM, Buehler JW, Hogue CJ, Strauss LT, Smith JC. Regional differences in birth weight-specific infant mortality. Public Health Rep 1987;102:138-45.

8. Hoffman HJ, Damus K, Hillman L, Krongrad E. Risk factors for SIDS: results of the National Institute of Child Health and Human Development SIDS Cooperative Epidemiological Study. Ann N Y Acad Sci 1988;533:13-30.

9. Peterson DR, Sabotta EE, Strickland D. Sudden infant death syndrome in epidemiological perspective: etiologic implications of variation with season of the year. Ann N Y Acad Sci 1988;533:6-12. 10. Cooperstock M, Wolfe RA. Seasonality of preterm birth in the Collaborative Perinatal Project: demographic factors. Am J Epidemiol 1986;124:234-41. 11. Keller CA, Nugent RP. Seasonal patterns in perinatal mortality and preterm delivery. Am J Epidemiol 1983;118:689-98.

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