The migration of placenta even after 30 gestational weeks is a risk factor for postpartum hemorrhage

Almost 10% of all pregnant women are diagnosed with placental location abnormalities (PLA), including placenta previa and low-lying placenta; however, most of PLA resolve as pregnancy process. Despite of placental migration, postpartum hemorrhage (PPH) is often experienced in those cases. The association between the timing of placental migration and the risk of PPH has remained unclear.

The aim of this study is to investigate the relationship between the timing of placental migration and the PPH, and to establish a cutoff value to accurately predict the risk of PPH in cases who underwent the placental migration during course of pregnancy.

This was a retrospective cohort study using electronic medical records in Osaka University Hospital (Japan). Patients diagnosed with PLA after 22 gestational weeks using transvaginal ultrasonography were eligible for inclusion. All patients delivered at our hospital between 2009 and 2022. Focusing on the cases in which placental migration (more than 2 cm away from the internal cervical os) were observed, the association between the timing of placental migration and the PPH was investigated only in vaginal delivery using the multivariate analyses with collecting covariates that could affect the PPH. Based on the results, receiver operating characteristic curves were generated to determine the optimal cutoff for the number of weeks of placental migration to predict PPH.

One hundred and forty-five patients were diagnosed with PLA after 22 gestational weeks and delivered at our hospital between 2009 and 2022. Seventy-six after-placental migration cases with successful vaginal delivery were analyzed. The median gestational age of placental migration was 32 weeks, and the median amount of hemorrhage during delivery was 777 mL. PPH (500-1000 mL) occurred in 34 cases (44.7%), >1000 mL in 26 (34.2%). In the univariate analysis, a positive correlation was observed between the timing of placental migration and the amount of hemorrhage (r=0.365, P<.01). Furthermore, multivariate analysis showed that the timing of placental migration is the most strongly influential covariate (standardized partial regression coefficient: 0.31, P=.0036) for predicting the amount of hemorrhage. In addition, the timing of placental migration was a significant risk for PPH (more than 500 mL) (adjusted unit OR 1.26 [95% CI: 1.07-1.49]). The receiver operating characteristic curve indicated that the moderate cutoff of the timing of placental migration to predict the risk of PPH (more than 500 mL) was 29 gestational weeks. In comparison between the groups in which placental migration was confirmed before 29 gestational weeks (n=19) and after 30 gestational weeks (n=57), the incidences of PPH (more than 500 mL) were significantly higher in the group in which placental migration was confirmed after 30 gestational weeks (52.6% vs 87.7%, P=.0012).

The timing of placental migration is positively correlated with the occurrence of PPH. In particular, cases in which placental migration is confirmed after 30 gestational weeks should be monitored carefully as high risk for PPH, as the risk of PPH is not eliminated.