In this study, we showed that the placenta area in S2 of the MH group was significantly higher than that of the group without MH, on the contrary, cervical area was significantly lower in PAS patients with MH than that of the group without MH. Our findings regarding a possible association between placenta area in S2, cervical area and the risk of MH in complete placenta previa patients with PAS may improve our ability to predict adverse clinical outcomes and to refine obstetric management in these cases.
One strength of our study was its clinically relevant population - PAS women were grouped in terms of the amount of blood loss during cesarean section, MH (estimated blood loss > 2000 ml) and non-MH groups (estimated blood loss ≤ 2000 ml). The placenta area in sector 2 and cervical area can also be measured, which may be used to recognize MH in patients with PAS. Another strength was our study allowed to assess the risk of MH in PAS patients before cesarean section and take well prepared for surgery. The limitations of this study were its retrospective nature, the small number of women enrolled, and the fact that it involved only a single institution. A larger and multi-center study is warranted to further confirm the findings in this study.
Complete placenta previa women with prior cesarean section are at increased risk of MH requiring an emergency cesarean delivery that are difficult to be dealed with for obstetricians. Therefore, it is necessary to assess perioperatively the risk of MH in those patients with either ultrasound or MRI investigations. Ultrasound is still an important means of diagnosing PAS, but MRI can provide more information about the depth of placenta invasion.14 To our knowledge, most of the studies focused on the influence of cervical length on maternal and infant perinatal outcomes in PAS patients,15,16 and the studies on cervical area in PAS were rare. In this study, we focused on addressing placenta area in S2 and cervical area as indicators that might help to anticipate MH during caesarean section.
An increased thickness of the lower placental edge had been reported by some to increase the risk of vaginal bleeding in patients with placenta previa.17 To stop MH in the uterus, not only the irrigation arterial pedicles of uterus, but also their anastomotic components at the bleeding sector should be managed. Knowledge of the blood supply of uterine sectors (S1 and S2) is a prerequisite to stop bleeding accurately and rapidly during surgery. The blood supply in S2 is more complex than that in S1, and the operating space in S2 is narrower, so the bleeding in the S2 is difficult to control, resulting in an increased risk of MH in PAS patients. The S1 (above the peritoneal reflection) and S2 (below the peritoneal reflection) regions of the uterus are supplied by different blood vessels in the pelvis. The S1 is supplied by ascendent branches of the uterine artery and the descendent branches of the ovarian artery, and the S2 is supplied by uterine, cervical, upper vesical, vaginal, iliac internal, and pudendal artery branches.8
The findings from this retrospective study showed that prenatal MRI has an excellent sensitivity and specificity in identifying MH during cesarean section. In this study using ROC curve if placenta area in S2 > 45 cm2 was taken as a cut-off, the risk of MH was significantly higher as compared to cases with placenta area in S2 below this threshold. PAS patients with placental accreta that occur in the S2 were at a higher risk of complications during surgery.18 The study of Palacios Jaraquemada et al. found that up to 87.6% of placenta implantation occurred in the S2 region of the lower uterine segment.13 Uterine ligature, occlusion, or embolization was very effective for uterine bleeding in the S1 region, and the efficiency was even more than 90%.19 Placental accreta in the S2 region was an important cause of postpartum hemorrhage in PAS patients, and even internal iliac vascular ligation was ineffective.20
Shorter cervical length was strongly associated with MH and adverse maternal-fetal outcomes in patients with PAS.10 Many studies had focused on the association between cervical length and adverse perinatal outcomes in PAS patients. In this study, cervical area was strongly associated with the development of MH during surgery. PAS patients with placenta previa with a short cervix have a higher risk of MH, which may be caused by the low position of the placenta leading to dilated lower uterine segment and cervix, and the lack of muscle tissue that can constrict the torn vessels after placental separation.21,22 In late pregnancy, the placenta previa tends to pull the cervix, which caused the cervical length to shorten and the cervical area to shrink.23 Some researchers had pointed out that shortened cervical length means placenta previa cannot attach smoothly to the lower segment of the uterus and the cervix, which increased the risk of MH.24 Cervical area includes the length and thickness of the cervix, which may be able to reflect the cervical condition of PAS patients more comprehensively to a certain extent.