Of the 206 cases of sudden unexpected death in infancy (SUD) 170 were linked with the maternity information and birth certificates on the Oxford Record Linkage files. Statistically highly significant correlations were demonstrated with low maternal age, high parity, and low social class, the last two associations being more marked among the mothers of infants who died after the 12th week.
Each case of SUD was then matched with three control livebirths for maternal age, parity, civil state of mother, social class, year and hospital of delivery, and as closely as possible for area of residence. A comparison of mothers of cases with those of the controls revealed a highly significant preponderance of women who were born outside the area, and no significant effect with maternal religion, previous pregnancy loss, or ABO blood group.
Other pregnancies occurring to the case and control mothers between 1965 and 1971 were also traced. It was shown that the infant dying an SUD was more likely to have been conceived within six months of the birth of his preceding sib.