Maternal predictors of perinatal mortality: the role of birthweight

Background Many maternal characteristics increase the risk for perinatal death. To locate potential sites for intervention, it is important to identify these risk factors and examine how much of the excess mortality is explained by infants' low birthweight. Methods Data on all newborns in Finland born between 1991 and 1993 (N = 199 291, of which 1461 were perinatal deaths) were obtained from the Medical Birth Register. Logistic regression analysis was used to adjust for background variables, both including and excluding infants' birthweight. The percentage reduction in odds ratios after adjustment for infants' birthweight was used to estimate the contribution of infants' low birthweight to the excess mortality. Results After adjusting confounding factors, increased risk for perinatal death was found for eight maternal characteristics. In the following the increased risk is given as odds ratios and the proportions of the excess mortality explained by infants' low birthweight are in parentheses: in-vitro fertilization 4.12 (>100%); earlier stillbirth 3.43 (87%); higher maternal age, from 1.21 to 3.08 (38-99%); maternal diabetes 2.87 (50%); lower socioeconomic status, from 1.30 to 1.70 (27-44%); smoking during pregnancy 1.45 (>100%); single mother 1.44 (50%); first birth 1.36 (75%). Conclusions Excess mortality due to maternal risk factors occurred mainly through their tendency to cause low birthweight. However, the excess mortality associated with low socioeconomic status, single motherhood, and diabetes was mediated by other mechanisms in addition to low birthweight.