BIRTH-ORDER AND RISK OF SUDDEN-INFANT-DEATH-SYNDROME - IS THE TRUE RELATIONSHIP NEGATIVE

The risk of SIDS has been reported consistently as being positively related to parity or birth order. However the reports in question have tailed to take into account the possible confounding effects of sibship size. In the present study it was reasoned that if this were done the relationship would be negative, not positive. In an analysis of births stratified by sibship size occurring in the years 1975-84 in the state of Oregon, it was found that the risk of SIDS in the age range 7-364 days did indeed tend to decrease with increasing birth order when sibship size was held constant. An expected tendency for SIDS rates to increase with increasing sibship size when birth order was held constant was also confirmed. This tendency is probably explained by a negative correlation between family size and socio-economic status. Non-SIDS rates behaved in a similar manner to SIDS rates. The tendency for the risk of SIDS to decrease with increasing birth order was more evident when births following a pregnancy interval of less than 12 months were excluded.