Maternal pre- and perinatal influences on secular birthweight trends in Tasmania: A retrospective analysis of all live singleton births 2005-2018

Tasmania has the highest rates of child and adult obesity in Australia. Birthweight, a key indicator of foetal nutrition in utero, is a crucial determinant of obesity in childhood, adolescence and adulthood. We explored birthweight trends and associations with maternal characteristics in live-born infants between 2005 and 2018 using regression analysis. The Tasmanian Data Linkage Unit provided the matched and deidentified perinatal and birthweight data of 81700 mother-infant pairs. While mean birthweight (3425 g to 3359 g) and the proportion of high birthweight (14.2% to 11.0%) decreased, there was an increase in the proportion of low birthweight neonates (4.8% to 6.5%) in the 14-year period. A downward shift in gestation length distribution, increased rates of caesarean delivery, hypertensive disorders, age > 35 years, assisted conception and changes in ethnic composition with increased numbers of indigenous mothers and mothers born outside of Australia partly contributed to this trend. Interestingly, the impact of striking increases in the rates of pre-pregnancy obesity and gestational diabetes mellitus, factors commonly associated with increased birthweight, were not observed in this study. Future studies should consider other maternal factors potentially contributing to these trends, including gestational weight gain and glycaemic control in diabetic mothers.