Objectives: A change in the child policy could trigger a new baby boom with changes in birth-related characteristics. Besides, the adaption of the medical system to the policy needs demonstrating. Design: Retrospective study. Setting: Chongqing, China, in a comprehensive institution. Population: A total of 51,062 pregnant women with 52,589 neonates were included. Methods: All medical records involving delivery were obtained. The records of care-providers were from the HR department. Main Outcome Measures: The temporal patterns of deliveries, births and care-providers were displayed. The additional births and care-providers’ workload was calculated. The mothers’ constitution was described and the influence factors of obstetrical characteristics were identified. Results: The policy-leading upwards trends of deliveries and births were obvious. The percentage of mothers aged 35 and over increased (10.1% to 10.8% to 15.8%, p<0.001) and that of multipara also rose (17.8% to 25.9% to 37.6%, p<0.001). This new constitution of mothers was associated with additional births, and with a concurrent variation in CS rate (60.4% to 52.3% to 56.3%) but not sex ratio. The workload of care-providers decreased during this period. Conclusions: The baby boom and following pregnancy changes after the shifts of policies is obvious. The target women with hyper-age and multiparity contribute considerably to the additional births and to the policy’s general effects on various outcomes. Funding: National Natural Science Foundation of China (No. 81771614 and No. 81771613), and the National Key Research and Development Program of China (No. 2016YFC1000407). Keywords: China’s Family Planning Policy; birth; obstetrical characteristics; pregnancy complications.