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.