Some studies have shown that women with a previous cesarean section, compared to women with a previous vaginal delivery, have an increased risk of retained placenta during a subsequent vaginal delivery. It is unknown whether this is mediated by anterior placental location, when the placenta might cover the uterine scar. The aim of this study was to evaluate whether the increased risk of retained placenta in women previously delivered by cesarean section is mediated by anterior placental location.
This is a population-based cohort study, with data from the regional population-based Stockholm-Gotland Obstetric Cohort, Sweden, from 2008 to 2014. The overall study population included 49 598 women with a vaginal second delivery, where adequate information about placental location from the second trimester ultrasound scan was available. For the main analysis, including the 3921 women with a previous cesarean section, we calculated the relative 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) with 95% confidence intervals (CI). In a second model, adjustments were made for maternal age, height, country of birth, smoking in early pregnancy, infant sex and in vitro fertilization.
In the overall study population, the rate of retained placenta at the second delivery was 2.0%. The proportion of women with a retained placenta was higher among women with a previous cesarean compared to those with a previous vaginal delivery (3.4% vs 1.9%; p< 0.0001). In the main analysis, including women with a previous cesarean section, the risk for retained placenta was not increased with anterior compared to non-anterior placental location (OR 0.84, 95% CI; 0.60 to 1.20). Adjustments did not affect the estimates in a significant way.
The increased risk of retained placenta in women previously delivered by cesarean is not mediated by anterior placental location.

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