Strengths and limitations
Our study has several strengths: (1)a population-based single-centered study, (2) covering a large period between 2013 and 2020, (3) Because all patients delivered in a medical institution, we have a complete and systematic review of all medical records. All patients were followed up 6 weeks after delivery and no serious complications were found after discharge. Also, the study is limited to Shanghai subjects and has limitations owing to the retrospective design. It only represents the level of developed regions in China. The situation in other parts of china is still unknown, so further research is needed to understand the generalizability of the study findings.