Trends and social patterning of birthweight in Sheffield, 1985-94

Abstract

Aim - To describe the trends in birthweight and their association with socioeconomic status in a 10 year birth cohort in Sheffield. Methods - Data for all live singleton births were extracted from the Sheffield Child Development Study (SCDS) database for 1985-94. Enumeration districts (EDs), derived from postcodes, were ranked into deciles by Townsend Deprivation Index (TDI), based on the 1991 census. Birthweight by 500 g group and mean birthweights for the city and by ED decile were calculated by year. X2 analyses for linear trend were calculated. The proportion of variance in birthweight explained by ED decile and raw TDI score was estimated by year. Absolute risk difference of birthweight <3500 g for the most compared with the least deprived deciles and proportion of births <3500 g statistically 'attributable' to social inequality were calculated. Results - The mean birthweight for all births increased by 34 g between 1985 and 1994. This difference is largely accounted for by a positive trend in births ≥3500 g and a negative trend in births of 3000-3499 g. Similar trends were noted across all ED deciles. Around 10% of the variance in birthweight was explained by area deprivation level. Absolute risk difference for births <3500 g was 12.5% for the 10 years (range 8.3-18.4). The proportion of births <3500 g statistically 'attributable' to social inequality for the 10 year period was 9.6% (range 4.3-15.5). Conclusions - Despite an overall increase in mean birthweight, large social differences persisted during the study period. An average of 350 births a year were not in the group (≥3500 g) bestowing most health advantage throughout the life course, as a result of social inequality.