The effect of the weekend on the risk of sudden infant death syndrome

The risk of sudden infant death syndrome (SIDS) is associated strongly with socioeconomic status. However, many infants who live in one socioeconomic environment, with its attendant level of risk of SIDS over the weekend, often are exposed to a different level of risk during the work week (because of day care for the infant). If the association between SIDS and socioeconomic status acts through the quality of supervision of the infant, then there could be an immediate change in the level of risk as the infant moves from home to outside care to home again. In this scenario, infants of economically disadvantaged parents would have a higher risk of SIDS over the weekend than they do during the week. On the other hand, infants of economically advantaged parents would be at lower risk over the weekend. Therefore, the relative risk of SIDS associated with the weekend (risk over the weekend vs risk during the work week) should be found to decrease as the number of years of maternal education (a surrogate for socioeconomic status) increases. Testing this prediction is the objective of the study.

Instances of SIDS in the postneonatal period (28-364 days) among the cohort of all infants born in the United States between January 1989 and December 1991 were analyzed. The number 798.0, taken from the International Classification of Diseases, was used to identify 14 996 cases of SIDS. Deaths among hospital patients were distinguished from all other deaths. The latter were divided into four categories: 1) death occurred in the emergency department; 2) the infant was dead on arrival at the emergency department; 3) death occurred at a residence; and 4) death occurred at some other place. Maternal education was divided into four categories: <12, 12, 13 to 15, and >/=16 years. The weekend ratio was defined as the ratio of SIDS cases on Saturday and Sunday (times 5) and Monday through Friday (times 2). The predicted trend in this ratio by maternal education was tested by applying a chi(2) test-for-trend.

The overall weekend ratio was 1.00, indicating that the risk of SIDS was no higher over the weekend than it was Monday through Friday. However, for infants of mothers with <12 years of education, the ratio was 1. 13. For infants of mothers with >/=16 years of education, it was 0. 55. The trend in the ratio as maternal education increased (1.13, 0. 99, 0.86, and 0.55) was highly significant (chi(2) = 74.2; 1 degree of freedom). Each of the four ratios, with the exception of 0.99, was significantly different from 1.00 (z = 3.74, 2.45, and 6.09, respectively). The ratios for infants of mothers with 13 to 15 and >/=16 years of education also were significantly different from each other (z = 4.57). For all causes of death combined (including the relatively small number of SIDS cases) among hospital inpatients, there was no significant trend in the weekend ratio as the level of maternal education increased. However, among deaths not attributable to SIDS or accidents occurring outside the hospital, there was a slight but significant declining trend (chi(2) = 8.4; 1 degree of freedom) The risk of an accidental death was highest over the weekend for all four maternal education categories. On an average working day, the risk of SIDS among offspring of mothers with <12 years of education was found to be 3.9 times greater than that among offspring of mothers with >/=16 years of education. At the weekend, the relative risk increased to 7.9. A plot of the weekend ratio against single years of maternal education revealed a unimodal distribution with a peak at 11 years.

First, the results of the study are consistent with the level of risk of SIDS, changing promptly toward the risk level obtained in the baby's new environment. Variability in the observation of unusual respiratory events seems the most likely explanation. It is unlikely that confounding factors played a role in the results for tertiary-educated mothers