Evidence-based clinical review (ACOG/SMFM, RCOG, FIGO, UpToDate) for the definition of placenta previa and placenta accreta spectrum, risk stratification, ultrasonographic diagnostic thresholds, multidisciplinary antenatal management, timing of birth, and principles of cesarean hysterectomy.
⚕️ Target audience: This clinical review is intended for physicians and healthcare professionals; It is a summary of current literature and guidelines for educational purposes. Clinical decisions should be made based on patient-specific evaluation and applicable institutional protocols.
Placenta previa is when the placenta covers the internal cervical os. In current terminology (RCOG GTG 27a, 2018; ISUOG), if the lower edge of the placenta is closer than 20 mm to the internal os but does not cover the os, it is called "low-lying placenta", and if it completely covers the os, it is called "previa". The "total/partial/marginal previa" classification of the previous generation has been abandoned in favor of these two measurement-based categories.
Placenta accreta spectrum (PAS) describes the abnormal depth of attachment of placental villi to the myometrium. Histopathological triple picture: accreta (villi adhere to the superficial myometrium in the absence of decidua basalis), increta (villi invade the myometrium) and percreta (invasion exceeds the serosa and reaches neighboring organs). PAS is preferred over the term "Morbidly adherent placenta" (ACOG/SMFM Obstetric Care Consensus No. 7, 2018, reaffirmed 2021; PMID 30461695).
FIGO clinical classification grades surgical-pathological findings (Jauniaux et al., Int J Gynaecol Obstet 2019;146(1):20–24; PMID 31173360):
The most clinically critical distinction is between merely adherent (Grade 1) and invasive (Grade 2–3) forms; because surgical planning, blood loss expectation, and risk of organ injury vary significantly on this axis.
The incidence of PAS is increasing in parallel with the increase in cesarean section rates; Contemporary population data report a range of approximately 1/272 to 1/500 pregnancies (0.15–0.94%) (FIGO 2019; Bonanni et al., JAMA Netw Open 2025;8(7):e2521909, PMID 40679824). Placenta previa occurs in approximately 0.5% of term pregnancies. Most of the low-lying placentas detected in the second trimester "migrate" until term (placental migration) with the elongation of the lower uterine segment; Persistence of previa is generally 10% or less and is proportional to the placenta-os distance in early pregnancy.
The strongest risk factor for PAS is the combination of previous cesarean section and existing previa. Silver et al.'s multicenter cohort (Obstet Gynecol 2006;107(6):1226–1232) provided baseline data showing that the risk of accreta in the presence of previa increases exponentially with the number of caesarean sections.
| Number of caesarean sections | Risk of accreta when previa is present |
|---|---|
| 1st (first) cesarean section | 3% |
| 2nd cesarean section | 11% |
| 3rd cesarean section | 40% |
| 4th cesarean section | 61% |
| ≥5th cesarean delivery | 67% |
In the overall cesarean cohort studied by Silver et al. (2006), the frequency of accreta was 0.24% at the first cesarean and 6.74% at the sixth or subsequent cesarean; these figures are not specific to the subgroup without previa. Other risk factors include previous uterine surgery (myomectomy, curettage, hysteroscopic procedures), in vitro fertilization/assisted reproductive techniques, advanced maternal age, multiparity, Asherman syndrome and a history of cesarean scar pregnancy (ACOG/SMFM OCC No. 7, 2018). This profile provides a simple but powerful clinical rule: previa together with at least one previous cesarean should be managed with a working assumption of PAS until proven otherwise.
The most accepted mechanism is a defect of the endometrial–myometrial interface—typically a previous cesarean section scar. Normal decidualization fails in this region; The villi attach to the myometrium at an abnormal depth without the decidua barrier and trophoblast infiltration deepens. Secondarily, marked neovascularization and uteroplacental vascular remodeling develop in the lower uterine segment (LUS); This vascular remodeling underlies both the findings on imaging and the tendency for massive bleeding at surgery.
Linking two dimensional gray-scale and Doppler markers to pathophysiology significantly increased the diagnostic value. Adu-Bredu et al. (Ultrasound Obstet Gynecol 2025;65(1):85–93; PMID 39676233) found that LUS remodeling findings were common to both high-grade PAS (91.8%) and uterine scar dehiscence (87.0%) and did not differentiate the two; however, it showed that findings of abnormal vascularity (uteroplacental vascular remodeling + serosal hypervascularity) were specific to high-grade PAS (aOR 116.0 for high-grade PAS when uteroplacental remodeling was fully positive; 95% CI 15.3–878.3; AUC 0.90; sensitivity 89.6%, specificity 90.0%). This distinction is important in practice: while scar dehiscence alone is not a justification for hysterectomy, high-grade PAS with abnormal vascularity calls for planned cesarean hysterectomy.
The first step of diagnosis is ultrasonography in all current guidelines. Every pregnant woman who has had at least one previous cesarean section with previa or low-lying placenta should be evaluated with targeted USG for PAS. The second trimester is the main timing; some guidelines recommend starting screening in the first trimester. The combined use of transabdominal and transvaginal approaches best demonstrates the placenta–os relationship and the uterovesical interface. Standard marker definitions were reconciled by the SMFM Ultrasound Marker Task Force in conjunction with AIUM/ACR/GOHO, with support from ACOG and ISUOG (Shainker et al., Am J Obstet Gynecol 2021;224(1):B2–B14).
| Modality | Marker | Description |
|---|---|---|
| 2D greyscale | Placental lacunae | Irregular, large, turbulent flow spaces (Finberg grade 3 significant); sensitivity ~77% |
| "Clear zone" loss | Disappearance of retroplacental hypoechoic zone; sensitivity ~66% | |
| Myometrial thinning | <1 mm + serosa protrusion / placental bulge (outside the uterine contour) | |
| Bladder line irregularity | Disruption at the uterovesical interface; exophytic mass = percreta; sensitivity ~50% | |
| Color Doppler | Uterovesical/subplacental hypervascularization | Abnormal vascular condensation |
| Bridging veins | Vertical veins originating from the placenta | |
| Lacuna feeding vessels | High speed flow (>15 cm/s); Color Doppler findings sensitivity ~91% |
The performance of combined USG is high: sensitivity approximately 90.7%, specificity approximately 96.9% (Bonanni et al. 2025 systematic review, PMID 40679824). A critical caveat: normal USG does not exclude PAS; Clinical risk factors are as predictive as imaging, so the absence of markers in a high-risk pregnancy should not give false confidence (ACOG/SMFM OCC No. 7). Low/scar level gestational sac implantation and cesarean scar pregnancy (CSP) in the first trimester are considered early continuation of PAS and considered an early warning sign.
MRI is not routine; It is valuable in selected scenarios: posteriorly located placenta, cases in which USG remains unclear, and presurgical mapping of invasion depth and parametrial/bladder spread. Intraplacental dark T2 bands, uterine "bulging", placental heterogeneity, focal myometrial interruption and abnormal vascularity are looked for on T2-weighted images. MRI has not been shown to be superior to USG; Its performance is largely dependent on center experience, and guidelines have not yet converged on contrast use, optimal timing, and its role in anterior low placenta (Bonanni et al. 2025).
The strongest recommendation determining the results in the management of PAS and with full agreement among the guidelines (100%) is to refer the patient to an experienced multidisciplinary reference center (Level III/IV maternal care) (Bonanni et al. 2025, PMID 40679824). The best results are achieved when delivery is planned before bleeding or labor begins and placental disruption is avoided; blood loss and morbidity differ markedly between planned surgery and emergency surgery.
Mandatory team components: maternal-fetal medicine/obstetrics, obstetric anesthesiology, an experienced pelvic surgeon, urology, interventional radiology, neonatology, and hematology/transfusion medicine. Essential infrastructure includes on-site adult and neonatal intensive care, a blood bank capable of massive transfusion protocols, and intraoperative cell salvage. When delivery is planned before 37 0/7 weeks, antenatal corticosteroids are administered for fetal lung maturation, timed to the anticipated delivery window (typically around week 34).
A review by Bonanni et al. comparing 14 guidelines showed high concordance (88.9%) on antenatal management recommendations, while revealing insufficient evidence and wide variability regarding anemia/iron supplementation and thromboprophylaxis strategies (57.1% insufficient evidence for postnatal iron and VTE prophylaxis). In practice, preoperative hemoglobin optimization is standard, whereas specific iron replacement and thromboprophylaxis regimens follow institutional protocols.
For planned delivery in uncomplicated, stable PAS, ACOG/SMFM OCC No. 7 recommends a window of 34 0/7 – 35 6/7 weeks (late preterm). In its separate publication, SMFM defines flexibility between 34 0/7 – 37 0/7 weeks; RANZCOG (C-Obs 20, 2023), RCOG and SOGC mostly agree on a 34–35 6/7 week window, with some open to extension up to 37 weeks. In the presence of indications such as bleeding, labor, premature rupture of membranes or preeclampsia, earlier emergency delivery is performed; Approximately 32% of patients with PAS deliver as indicated (most common cause, bleeding in 65%) before the 36th week (Rosenthal et al., Am J Obstet Gynecol 2025;233(3):201.e1–11; PMID 39983888).
Current evidence—late birth applicable in selected cases: In Rosenthal et al.'s 202-patient cohort (AJOG 2025; PMID 39983888), planned delay of birth beyond 36 0/7 weeks in a specialized center geographically close to the patient in stable PAS with no other indications improved neonatal outcomes without worsening maternal outcomes (fewer NICU admissions, RDS, and mechanical ventilation; median 37 weeks vs early group 35 weeks). PAS severity did not differ between groups. This data is a center-dependent finding that calls into question the uniform application of the narrow 34–35 6/7 window; It is not yet a guideline change and should only be considered in centers where rapid surgical access is assured in the absence of all urgent indications.
The most widely accepted approach is a planned cesarean hysterectomy: after the fetus is delivered, the placenta is left in place and no attempt is made to remove it. Manual or forced removal of the placenta triggers massive bleeding and is not recommended. Bonanni et al. (PMID 40679824) identified common elements in surgical principles with high interguideline agreement (88.9%):
Areas of disagreement include choice of anesthesia (general vs regional—case by case), type of skin incision, and prophylactic ureteral stent use. In one study, the stent reduced the likelihood of genitourinary injury by approximately 80% (PMID 36201777), while in another study it failed to show a protective benefit and was associated with more postoperative urological complications (PMID 37403369); The evidence is mixed. The routine benefit of prophylactic internal iliac/uterine artery balloon occlusion has also not been proven.
Conservative (uterus-sparing) management can be applied in selected cases—especially when single-stage resection is possible in focal/central disease—by leaving the placenta in situ. Adjuvant methotrexate is not recommended. This approach is for patients who want fertility preservation, are hemodynamically stable, and are in an experienced center; There is insufficient evidence for the risk of recurrent PAS and future pregnancy rates (33.3% insufficient evidence for conservative techniques; Bonanni et al. 2025). Surgical morbidity—cystectomy, bowel/ureter injury, massive transfusion, need for intensive care—increases in proportion to the number of previous cesarean sections (Silver et al. 2006).
Bleeding management is the area where evidence is weakest in the guidelines (55.6% insufficient evidence in Bonanni et al. 2025); Most recommendations are based on expert opinion and extrapolation from the non-pregnant population. In contrast, there is robust randomized evidence for tranexamic acid. The WOMAN study (Lancet 2017;389(10084):2105–2116; PMID 28980954) evaluated a regimen of 1 g TXA IV (100 mg/mL over 10 minutes) for postpartum hemorrhage in 20,060 women from 21 countries; The evaluated regimen included a second 1 g dose if bleeding persisted after 30 minutes or recurred within 24 hours. TXA reduced death from hemorrhage from 1.9% to 1.5% (p=0.045), and the benefit was greatest when given within the first 3 hours after birth (1.7% → 1.2%; p=0.008); No significant benefit was seen after 3 hours. WHO and FIGO recommend that TXA be administered as soon as postpartum hemorrhage is diagnosed and within ≤3 hours after birth.
Accompanying guidelines: massive transfusion protocol with balanced ratio (e.g. 1:1:1 erythrocyte:plasma:platelet), fibrinogen replacement (cryoprecipitate/concentrate; target >2 g/L), calcium supplementation and prevention of hypothermia; Intraoperative cell salvage is recommended in PAS. Interventional radiology options (prophylactic internal iliac/uterine artery balloon occlusion or, in selected high-grade cases, resuscitative endovascular aortic balloon occlusion in infrarenal Zone III—REBOA) may be used to reduce blood loss; It is not routine, it is source and center dependent, and randomized evidence is limited. In bleeding due to previa (non-PAS), uterine/internal iliac artery ligation and compression sutures are tried step by step, while in PAS, by definition, early hysterectomy is performed.
Last updated: . Guide numbers and publication years have been verified against current catalogues.