Factors and outcomes for placental anomalies: An umbrella review of systematic reviews and meta-analyses

Background Placental anomalies, including placenta previa (PP), placenta accreta spectrum disorders (PAS), and vase previa (VP), are associated with several adverse foetalneonatal and maternal complications. How-ever, there is still a lack of robust evidence on the pathogenesis and ad-verse outcomes of the diseases. Through this umbrella review, we aimed to systematically review existing meta-analyses exploring the factors and outcomes for pregnancy women with placental anomalies. Methods We searched PubMed, Embase, Web of Science, and the Co-chrane Library from inception to February 2023. We used AMSTAR 2 to assess the quality of the reviews and estimated the pooled risk and 95% confidence intervals (CIs) for each meta-analysis. Results We included 34 meta-analyses and extracted 55 factors (27 for PP, 22 for PAS, and 6 for VP) and 16 outcomes (12 for PP, and 4 for VP) to assess their credibility. Seven factors (maternal cocaine use (for PP), uterine leiomyoma (for PP), prior abortion (spontaneous) (PP), threatened miscarriage (PP), maternal obesity (PP), maternal smoking (PAS), male foetus (PAS)) had high epidemiological evidence. Twelve factors and six outcomes had moderate epidemiological evidence. Twenty-two factors and eight outcomes showed significant association, but with weak credibility. Conclusions We found varying levels of evidence for placental anoma-lies of different factors and outcomes in this umbrella review.