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OC30.05: Placental location and risk of retained placenta in women previously delivered by Caesarean section: a population‐based cohort study

Objectives: Women with a previous Caesarean section, compared to women with a previous vaginal delivery, have an increased risk of retained placenta during a subsequent vaginal delivery. The aim of this study was to evaluate whether this increased risk is mediated by anterior placental location, as an anterior placenta might be attached to the uterine scar. Methods: This is a population-based cohort study, with data from the regional Stockholm–Gotland Obstetric Cohort, Sweden. All women who delivered their second child vaginally during the years 2008 – 2014, and had adequate information about placental location from the early second trimester ultrasound screening during the second pregnancy, were included (n = 49 598). Placental location during the second pregnancy and the risk of retained placenta at second delivery were calculated in relation to previous Caesarean section or not. For the main analysis, only the 3 921 women with a previous Caesarean section were included. We calculated the risk of retained placenta in women with an anterior placental location, using women with non-anterior placental locations as reference. Relative risks were calculated as odds ratios (OR). Results: An anterior placental location was slightly more common in women with a previous vaginal delivery compared to women with a previous Caesarean section (46.9% and 45.1%, respectively [p = 0.03]). The overall rate of retained placenta at the second delivery was 2.0%. The proportion of women with a retained placenta was higher among women with than without a previous Caesarean section (3.4% vs 1.8%; p < 0.0001). In women with a previous Caesarean section (n = 3 921), the relative risk for retained placenta was not increased in women with anterior (OR 0.8; 95% CI 0.6-1.2) compared to non-anterior placental location. Conclusions: The increased risk of retained placenta in women previously delivered by Caesarean section is not mediated by anterior placental location at the early second trimester ultrasound screening.