Multiple abortions and intrauterine procedures may damage the endometrium, and placental villi can penetrate the myometrium, leading to placental implantation. Previous cesarean section is an independent risk factor for PP and placenta implantation[18]. The normal placenta is attached to the decidua basalis of the uterus, and the placenta can be removed smoothly after delivery. PP is often accompanied by placental percreta, in which villi penetrate through the entire myometrial thickness or surrounding organs[19]. When the placenta inserted into the myometrium is removed, the local myometrium is missing and leading to massive bleeding. The damage of myometrial by the placental increta is responsible for maternal bleeding and potential fetal compromise[20]. PP often leads to uncontrolled bleeding during childbirth or postpartum, which can cause serious consequences, even life-threatening. About 40-60% of the peripartum hysterectomies are due to placenta increta[21]. The IPH group had a higher NICU admission, mainly due to cesarean sections at a smaller gestational age in IPH group. Therefore, prenatal diagnosis of placental implantation and prediction of intraoperative blood loss can help clinicians make adequate preoperative preparation, develop appropriate surgical procedures and avoid serious complications.
Although ultrasound is an important method for the diagnosis of PP, MRI has been used more and more in the diagnosis and treatment of PP in recent years[22, 23], which fully demonstrates its value in the evaluation of intraoperative blood loss of PP. The aim of our study was to investigate the role of MRI for the PP diagnosis and the clinical prediction in IPH. Our current study included 125 cases of PP at high risk of co-existing placental accreta. When the patient underwent MRI examination, it was better to have about 400 ml of urine in the bladder, which was beneficial to predict whether the placental tissue was implanted into the bladder. In this study, blood loss was measured by using weighed swabs, which is more precise than methods using visual estimation. Patients with PP have more intraoperative bleeding due to the intense bleeding of the uterus during delivery of the placenta, especially those with the placenta located entirely in the lower uterine segment. Placental implantation can cause the placenta and uterine wall contact closely, postpartum placenta is not easy to peel and affect the uterine contraction, resulting in uterine blood sinus can not be closed and postpartum hemorrhage.
In MRI images, the typical shadowing characteristics of placenta implantation include thinning of the myometrium, placental penetration into the cervix, and uneven placental signals. Interruption of myometrium signal and placental invasion into pelvic tissues and organs are the most direct manifestations of placental implantation in MRI. However, due to the thinning of myometrium in the third trimester of pregnancy, the above features are lack of sensitivity and difficult to visualize, so they are rarely used in clinical diagnosis[24]. In this study, indirect signs (palcenta position, placenta thickness, bladder line, and placenta pit, etc.) were combined with the imaging characteristics of placenta previa for overall analysis, which was beneficial to improve the reliability of clinical diagnosis. Our study demonstrated that placental position, placental thickness, cervical blood sinus and placental signals in the cervix were independent predictors in predicting the risk of IPH. The intraplacental blood sinuses may be an overgrowth of the placenta and inserted into the myometrium[25]. Our study found that placental signal in the cervix (OR = 10.913 ) and cervical blood sinus ( OR = 7.519 ) are the two major MRI signs at high risk of intraperitoneal bleeding, because it may indicate placental implantation in the cervix. Anterior to the lower segment of the uterus is the bladder, left and right ureters, and posterior to the rectum, which presents a great challenge to the operation of hemostasis in the cervix. An increase in the thickness of the placenta usually indicates that the placenta's blood supply is abundant, even implanted in the uterine wall. The placenta anteriorum is another risk factor for intrapartum bleeding, especially in pregnant women with a history of cesarean section. The placenta is easily implanted and can even penetrate the bladder, causing intrapartum bleeding and bladder damage.
Our study indicated that MRI-based nomogram could provide a non-invasive way to predict the risk of IPH in PP, which was confirmed by calibration and decision curve analyses. Our study showed that combining multiple MRI features has higher diagnostic value than a single feature, with high AUCs in the training and validation set. If a pregnant woman has a high risk score, abdominal aorta, common iliac artery balloon occlusion and other procedures may be selected for cesarean section. All cesarean sections in PP patients were performed by professional senior physicians, and thus had a lower hysterectomy rate (1.60%). The majority of newborns in the IPH group were transferred to the NICU, mainly due to the higher rate of premature births in this group. The nomogram of the combined model might be an effective and easy-to-use tool to estimate the danger level of PP before surgery, and patients might be given adequate preoperative evaluation and preoperative communication.
Despite the promising results, our study also had several limitations. Firstly, Our retrospective data were collected from a single unit for training and validating the predictive model. Secondly, our sample size is small, so we still need to increase the sample size for verification.