How common is repeat sudden infant death syndrome?

Recurrence of sudden infant death syndrome (SIDS) is rare but may give rise to confusion and controversy because of the differential diagnoses of familial disease or covert homicide. We examine eight studies of recurrent SIDS published in English since 1970. These studies reported relative risks of recurrence, as compared with the population or with controls, ranging from 1.7 to 10.1. We assess the validity of the studies by three main criteria: accuracy of ascertainment, adequacy of investigation and matching of controls. We found that all the studies failed to meet these criteria, and we think that their flaws would have resulted mainly in overestimation of recurrence risk. We conclude that, although an increase in risk is probable on theoretical grounds, this risk cannot be quantified from the available evidence. We suggest that professionals should be cautious in their pronouncements on the chances of recurrence, and that parents who have lost a baby to SIDS can, with the exception of particularly vulnerable families, be reassured that the risk of recurrence is small.