The rapid decrease in SIDS incidence between 1992 and 1995 paralleled an equally rapid decrease in the use of the prone sleeping position. At the same time, a parallel decline in post neonatal mortality was seen.4
A more gradual decrease in SIDS incidence was observed between 1995 and 2000, with a levelling off between 2000 and 2004. The lack of further decline in SIDS incidence after 2000 may be because it is possible that the main decrease in prone sleeping position occurred before 2000. However, the gradual decline in smoking during pregnancy has been continuous, and we do not know why this is not reflected in a corresponding decrease in SIDS incidence in recent years.
The decrease in SIDS incidence was paralleled by a decrease in prone sleeping between 1995 and 2004. However, during this period smoking during pregnancy also decreased, which it did not between 1992 and 1995, suggesting that smoking behaviour is more resistant to campaigns.
The prevalence of side and an alternating side/supine position during sleep is still high and is rising in Sweden. Several studies have found an increased risk of SIDS with the side position,5,13
and a recent publication from the CESDI study14
reports more than multiplicative risks when the side position is combined with birth weight <2500 g. As in numerous other countries, the side position is advocated in many maternity wards in Sweden, supposedly for fear of aspiration. This is a matter of concern, and these findings on SIDS need to be communicated to maternity ward staff.
Since the prone position is preferred by 5.6% of families in Western Sweden, in spite of the well known dangers, we examined why this position is still used. In a univariate analysis we found several factors to be significantly associated with the choice of prone position at 6 months of age, and these were then analysed in a multivariate model. However, in both the univariate and the multivariate analysis, the factor with the strongest association with prone position at 6 months was that the infant was placed prone to sleep in the first week. When the same analyses were repeated with prone position in the first week as the dependent variable, we found that the choice of prone sleeping position is determined by maternal employment status, maternal smoking, temperament of the child, dummy use, and sharing a bedroom with other children. However, these results must be viewed with caution, since only 133 infants slept prone, which is fewer than the number of missing answers regarding sleep position at 1 week (137 were missing).
What is already known on this topic
- Prone sleeping and maternal smoking are major preventable risk factors for SIDS.
- In Sweden, prone sleeping decreased, supine sleeping increased, and smoking during pregnancy remained unchanged during the first years after a campaign was launched to reduce the risk of SIDS.
- A rapid decrease in SIDS incidence occurred between 1992 and 1995, paralleling abandonment of the prone sleeping position, followed by a slower decrease between 1995 and 2000.
It is well known that infants sleep better in the prone position and when they use a dummy. Therefore, we believe that infants not using a dummy, especially fussy infants, might be placed prone to make them sleep better. In a bedroom with older children, because of the possibility of disturbance, it might also be easier for a small infant to sleep when prone. Why maternal unemployment increases the risk is a more difficult question. It is possible that unemployment reflects a social situation with increased strain on the family, which might lower the threshold for using the prone position. The infants might also belong to families with difficulties adhering to official recommendations and thereby with a higher prevalence of other risk factors. In these families, especially with infants with insomnia or intensive crying, it might be appropriate to reinforce the advice on the supine sleeping position.
We found a statistically significant difference in alcohol habits between the Western Sweden study and the Nordic Study. High alcohol use during pregnancy and high use after pregnancy both decreased. In a recent study in Sweden,15
it was found that approximately 30% of pregnant women reported regular alcohol use during pregnancy, but only 2/1101 (that is, 0.18%) reported drinking two to three times a week. This is in accordance with the figures from the Western Sweden study.
Alcohol figures must be viewed with caution since there is a high risk of under‐reporting the use of alcohol in studies of this type.16,17
On the other hand, it is quite possible that under‐reporting is higher when the pregnant woman is personally interviewed by her midwife, which is how official statistics concerning alcohol use in pregnancy are obtained.
The reason for the fall in vitamin A and D supplements is unclear.
Consideration must be given to the fact that the study design differed between the two studies. All the infants in the cohort from Western Sweden were 6 months old. The age of the control infants in the Nordic Study varied between 2 and 57 weeks, with a mean of 19 weeks. This age difference can certainly influence the prevalence of factors such as sleeping position, but the trend from prone to alternating side/supine and exclusively supine seen in the Western Sweden study was also demonstrable during the years of the Nordic Study.4
Direct comparison of breast feeding figures is also restricted by the control infants in the Nordic Study being on average younger than the infants in Western Sweden, but the finding of an unchanged prevalence is supported by official statistics.18
What this study adds
- The decrease in SIDS incidence levelled out in 2000.
- Prone sleeping and maternal smoking during pregnancy decreased between 1995 and 2004. Variants of side sleeping are still used by 44% of parents.
- Prone sleeping is associated with a disadvantaged background or an infant with sleeping difficulties. Changing the sleeping position to exclusively supine and reducing smoking by pregnant women will further reduce the risk of SIDS.
An important issue in comparing the data was the response rate. In the Western Sweden study, the response rate was 68.5% among parents of 6 month old infants. Considering that this study was population based, it is reasonable to compare it with the response of the controls in the Nordic Study, which had a 72% response rate. The Nordic Study found odds ratios for important risk factors similar to those calculated from data in the Medical Birth Registry.19
We have compared the prevalence of smoking during pregnancy (9.5%) in the Western Sweden study with recent statistics from the National Board of Health and Welfare, showing that 11.4% of all pregnant women in Western Sweden smoked in 2002 compared to 10.6% in Sweden as a whole. There is a trend towards a lower prevalence of smoking during pregnancy.20
We were concerned that families from disadvantaged backgrounds might be under‐represented, but since smoking is linked with several disadvantageous factors, we believe that the data from Western Sweden are representative.
In conclusion, we have shown that parents of infants have in general complied with advice given at infant welfare centres to prevent sudden infant death and that the information is still effective 10 years after it was first introduced. Additional gains can be made by changing the preferred sleeping position from side and side/supine to exclusively supine and reducing the number of pregnant women smoking. We have also found that sleeping position is associated with whether the mother is unemployed, whether she smokes, and whether the infant is irritable, never uses a dummy, or shares a bedroom with other children.