Abstract
BACKGROUND: As population aging intensifies and women's fertility levels decline continuously, the improvement of fertility policies has emerged as a pivotal concern for most governments. This study aimed to evaluate the effect of the "universal two-child" policy implementation on the birth population trend. METHODS: A quasi-experimental interrupted time series (ITS) study was conducted to analyze the collected data. Data on the birth population of Shandong province from 2000 to 2022 were collected to observe trend changes before and after the intervention. RESULTS: The birth rate increased immediately in the first year after the intervention (p < 0.001), but the trend significantly declined in the following years (p < 0.001). After further adjustment for the influence of the pre-fertility policies, urbanization rate, and per capita disposable income, the birth population level still showed a downward trend year by year in the post-intervention period (p < 0.001). CONCLUSION: The policy implementation helped to boost the population growth in the short term, but the long-term effect of the policy was not optimistic. More targeted incentive strategies should be considered to reverse the continuous decline in fertility rates.