Association between postterm birth and adverse growth outcomes in children aged 3-6 years: A national retrospective cohort study.

BACKGROUND Gestational age significantly influences children's growth and development. Yet, the effect of postterm birth (gestation beyond 42 weeks) on children's growth outcomes remains underexplored. OBJECTIVES This study aimed to assess the impact of postterm birth on adverse growth outcomes in children using a nationally representative sample from China. METHODS A retrospective cohort study was conducted in China from 1 April 2018, to 31 December 2019. The final analysis included 141,002 children aged 3-6 years from 551 cities. Postterm birth was defined as children with postterm birth at a gestational age of 42 weeks or more. Obesity, overweight and thinness were assessed using body mass index-for-age (BMI-for-age) z-scores, based on the World Health Organization (WHO) Child Growth Standards. Generalised additive models were employed to investigate the non-linear relationship between maternal gestational age and BMI-for-age z scores. Poisson regression models and subgroup analyses with forest plots were performed to examine the associations between postterm birth and the risks of obesity, overweight and thinness in children. RESULTS We included 141,002 mother-child pairs, of whom 7314 (5.2%) children were classified as postterm births. There exists a non-linear relationship between gestational age and BMI-for-age z scores. Children born postterm exhibited a 46% increased risk of obesity, a 27% increased risk of combined overweight/obesity and a 13% increased risk of thinness. Similar associations were observed in most cases when further sensitivity and subgroup analysis were conducted. CONCLUSIONS Postterm birth was associated with elevated risks of obesity, overweight and thinness in children aged 3-6 years, independent of sex. These findings underscore the importance of further research across diverse populations to understand the implications of postterm births on child health outcomes.