Assessing the impact of China's universal two-child policy on infant health: evidence from a quasi-experimental study

China's "one-child policy" limited many households in China to only one child. This policy had an impact on birth outcomes due to the birth order effects, as firstborn infants typically have lower birth weights. This study aimed to estimate the impact of the "universal two-child policy" on birth weight in China by analyzing individual-level data collected from a major tertiary obstetrics hospital located in Shanghai, the largest metropolitan area in China. Medical records for all births were obtained from a major metropolitan obstetrics hospital between 2013 and 2018. Using difference-in-differences (DID) and quantile DID (QDID) methods while controlling for maternal characteristics and socioeconomic factors, we examined the policy's impact on birth weight. Analyses included stratification by maternal migrant status, age, and delivery mode. Insurance was found to mediate the treatment effect significantly. Analysis of 133 358 live births showed the policy increased birth weight by 21 g, corresponding to approximately 0.04 standard deviations of birth weight in our sample, with effects varying across maternal age groups and residency status. Insurance coverage mediated 41.3% of the total effect on birth weight. The "universal two-child policy" demonstrated beneficial impact on birth weight in China during the study period, particularly affecting older women, Shanghai residents, and those with natural births.