Risk Factors for Sudden Infant Death Syndrome Following the Prevention Campaign in New Zealand: A Prospective Study

DISCUSSION

Ninety-two percent of newborn infants are seen by Plunket nurses in New Zealand (unpublished data), and 77% of all children born in the 2-year cohort participated in the study. However, the response rate of SIDS cases was significantly lower than that of controls. Indeed refusal to participate in the study or lack of supervision by community child health nurses were significantly associated with an increased risk of SIDS. A previous report from the Christchurch region of the study also found that nonparticipants were more likely to smoke than participants.¹⁴ In this national study those who did participate differed from all live births in New Zealand. Mothers in participating families were more likely to be married and not to have smoked in pregnancy than expected. However, mothers' age and the infants' birth weight, gestation, and ethnicity did not differ. This means that caution is required to extrapolate the results to the whole community. However, as measurements were made before the categorization into case and control, there is no differential selection bias; hence, the results are valid within this sample.

New Zealand was the first country to formally launch a national SIDS prevention campaign, although earlier work from Holland had suggested that SIDS mortality might be reduced by the recommendation to avoid placing the infant prone to sleep.^15,16 This campaign resulted in a rapid decline in the prevalence of the prone position and an associated decrease in SIDS mortality.^1,7

Recall bias is a potential problem with retrospective studies. In view of the publicity surrounding the prevention campaign, recall bias might be expected to be a major problem. Collecting data prospectively, as in this study, avoids this problem. This report is the first national study of risk factors for SIDS following a national SIDS prevention campaign. The previously identified major sociodemographic, pregnancy, and infant risk factors were significantly associated with an increased risk of SIDS.

Despite the fact that the prevalence of prone sleeping position has fallen to a low level (0.7% and 3.0% at initial and 2 months, respectively), it continues to be a significant risk factor for SIDS. We previously calculated that prone sleeping position was causally associated with 52% of SIDS cases in New Zealand in 1987/88.⁴ We now estimate that prone sleeping position is associated with 10% of SIDS cases. This indicates the importance of maintaining the advice on infants' sleeping position in the national SIDS education program.

Most infants are now placed to sleep on their sides in New Zealand. We have previously reported that the risk of SIDS is greater in the side sleeping position compared with the supine position; however, the result was not statistically significant after controlling for potential confounders.⁵ This study found a statistically significant increased risk for SIDS with the side sleeping position after adjustment. A similar finding has recently been reported from the United Kingdom.¹⁷ A further substantial reduction in SIDS may occur with a change to the supine sleeping position. It now seems appropriate to advise the back sleeping position rather than the side or back sleeping positions.

We previously reported that breastfeeding was associated with a reduced risk of SIDS.¹⁸ Although breastfeeding was not found to be protective in this study, this result should be interpreted with caution as breastfeeding rates are high in New Zealand and this study might lack the power to detect a benefit. Furthermore, in countries with a low breastfeeding rate the potential benefit on SIDS mortality will be much greater.

Bed sharing was again identified as a risk factor for SIDS. There are now 10 studies that have examined the association between bed sharing and SIDS.^5,17,19-25 As our data were recorded prospectively, recall bias cannot be an explanation for this result. We have previously reported that the increased risk of SIDS with bed sharing was predominantly for infants of mothers who smoked.^8,9 This study and two other studies confirm that finding.^17,25 No statistically significant increased risk with bed sharing was seen for infants of mothers who did not smoke. As the bed sharing measure referred to a practice that occurred previously (sometimes months before), we cannot exclude the possibility that bed sharing on the night of death is a risk factor. Mothers of infants should be advised that bed sharing substantially increases the risk of SIDS if she smokes. For mothers who do not smoke the evidence is inconclusive; however, there is no evidence that bed sharing in any group is protective.

PAR calculations for the joint exposure to bed sharing and smoking indicates that approximately 30% of SIDS cases might be prevented if either bed sharing or maternal smoking was avoided. This proportion may be even greater than 30% as a higher proportion of nonparticipants in the study are likely to have been bed sharing smokers.

There have been many studies showing an increased risk of SIDS associated with maternal smoking and we have argued that maternal smoking is causally related to SIDS.²⁶ We and others have shown fathers who smoke also increase the risk of SIDS, however, this increased risk is predominantly for infants of mothers who smoke.^27-29 Maternal smoking either alone or in combination with bed sharing is the major risk factor for SIDS in New Zealand. Smoking behavior has proved difficult to change. Indeed the prevalence of smoking in pregnancy does not appear to have changed in New Zealand.³⁰ This suggests the need for other strategies, including enforcing laws that prohibit the sale of tobacco to minors, minimum package size, restricting smoking in public places, plain packaging, stronger health warnings, and banning advertising and sport sponsorship.

Although SIDS mortality rates have declined in many countries with the reduction in the prevalence of the prone sleeping position,^1,15,31-34 this study indicates that further substantial reductions in SIDS mortality might be achieved with a change to back sleeping position, a reduction in maternal and paternal smoking, and a reduction in the joint exposure to bed sharing and maternal smoking.