剖宫产史孕妇产前子宫下段超声厚度与产后出血风险

原文

BACKGROUND: Women with a previous cesarean delivery have a higher risk of postpartum hemorrhage, including after successful vaginal birth after cesarean. Mechanisms underlying this excess risk remain unclear and may involve impaired myometrial contractility in the lower uterine segment or abnormal placental separation. OBJECTIVE: To evaluate the association between antenatal lower uterine segment thickness and postpartum hemorrhage among women with prior cesarean delivery who achieved vaginal birth after cesarean, and to explore whether this association differs according to the presumed mechanism of hemorrhage: atonic versus retained placenta. STUDY DESIGN: This was a secondary analysis of the multicenter randomized LUSTrial. We included women with a measurable lower uterine segment thickness who attempted labor and delivered vaginally. Stabilized inverse probability weighting was used to address selection bias due to conditioning on vaginal birth. Logistic regression models, overall and stratified by mechanism of postpartum hemorrhage (atony vs abnormal placental separation), were performed. RESULTS: Among 841 women with vaginal birth after cesarean and available lower uterine segment measurements, the median antenatal lower uterine segment thickness was 5 mm. Overall, each 1-mm increase in lower uterine segment thickness showed a trend toward lower odds of postpartum hemorrhage (weighted OR: 0.92; 95% CI: 0.78-1.03), though this did not reach statistical significance. In stratified analyses, lower uterine segment thickness was inversely associated with atonic postpartum hemorrhage (OR: 0.91; 95% CI: 0.84-0.99), whereas no protective effect was observed for postpartum hemorrhage due to abnormal placental separation (OR: 1.15; 95% CI: 0.73-1.82; heterogeneity p=.05). CONCLUSION: In this exploratory analysis, a thicker lower uterine segment was associated with a lower risk of atonic postpartum hemorrhage but not with postpartum hemorrhage caused by abnormal placental separation, consistent with distinct physiologic pathways after vaginal birth after cesarean. These findings are hypothesis-generating and should be confirmed in larger studies El resumen está disponible en Español al final del artículo.