Strengths and limitations
Our study has several strengths. First, all the patients were have their prenatal examination and delivered at our institution, where the clinical data were relatively complete. Second, our research had a large sample and a long research time span, which would be more representative. However, there are some limitations to this study. Due to the impact of retrospective analysis, the three groups of people had heterogeneity, and there is subjectivity in the selection of placental pathology, which may lead to bias. In addition, this study lacks a normal placental control group , and further prospective randomized controlled trials are still needed in the future.