⚠️ Placenta Accreta Spectrum Surgical Planning Simulator

Risk Factors: Previa Meets a Scarred Uterus

Placenta accreta spectrum (PAS) describes a continuum of abnormal placental attachment — accreta, increta, and percreta — in which chorionic villi attach to or invade through the myometrium because the decidua basalis is deficient. The single dominant driver of this deficiency is prior uterine surgery, above all cesarean delivery, and risk rises steeply once a low-lying or previa placenta implants directly over the old scar.

• ~1:300: PAS incidence, all pregnancies (rising with cesarean rate)
• ~3%: Risk: previa + 1 prior CS (first cesarean scar)
• ~40%: Risk: previa + 3 prior CS (published series vary)
• ~60%+: Risk: previa + ≥4 prior CS (approaches near-certainty)

Why the scar matters

Normal placentation depends on an intact decidua basalis, the endometrial layer that restrains trophoblastic invasion to a physiologic depth. A cesarean scar replaces this layer with fibrous tissue that lacks a functional decidual barrier. If the placenta implants over that scar — which happens preferentially when the placenta is low-lying or previa — chorionic villi can attach directly to myometrium (accreta), invade into it (increta), or breach the serosa and adjacent organs (percreta).

Each additional cesarean delivery enlarges the area of vulnerable scar tissue and increases the probability that a subsequent placenta will implant over it. Risk also rises with a shorter interval since the last cesarean, prior uterine curettage, myomectomy, endometrial ablation, and prior manual placental removal.

The combination of anterior placenta previa with two or more prior cesarean deliveries should trigger a formal PAS ultrasound protocol at a referral center — this pairing alone is the strongest single predictor in every major published risk model.

The rising incidence curve

PAS incidence has increased roughly ten-fold over the past four decades, tracking almost exactly with the global rise in cesarean delivery rates. Contemporary population estimates range from 1 in 300 to 1 in 500 pregnancies in centers with high cesarean rates, compared with roughly 1 in 2,500 in the 1980s.

Because the risk compounds with each subsequent cesarean, PAS has become a central argument for limiting primary cesarean delivery to accepted indications and for offering a trial of labor after cesarean (TOLAC) where appropriate — every avoided cesarean lowers a patient's lifetime PAS risk in future pregnancies.

Additional and less common risk factors

While previa-on-scar accounts for the large majority of cases, PAS can also occur without either classic feature:

• Advanced maternal age and multiparity independent of surgical history • In vitro fertilization / assisted reproduction (altered endometrial receptivity) • Asherman syndrome and prior severe endometritis • Prior uterine artery embolization • Bicornuate or septate uterus with abnormal implantation

Any patient with a low-lying placenta and any prior uterine surgery deserves a deliberate, documented assessment for PAS rather than reliance on classic risk factors alone.

Ultrasound and MRI Features by Severity

Prenatal diagnosis of PAS relies primarily on transabdominal and transvaginal grayscale plus color Doppler ultrasound performed by an experienced sonographer. MRI is a valuable adjunct — not a first-line test — reserved for posterior placentation, maternal obesity limiting acoustic windows, or suspected parametrial/bladder invasion where surgical mapping needs more detail.

• ~90–95%: Ultrasound sensitivity (experienced centers)
• ~65–95%: Ultrasound specificity (operator-dependent)
• 28–32 wk: Optimal screening window (repeat after 18–24 wk survey)
• <1 mm: Myometrial thickness cutoff (suggests deep invasion)

Grayscale sonographic signs

• Multiple irregular placental lacunae ("Swiss cheese" appearance) — vascular spaces within the placenta that increase in number and irregularity with severity • Loss of the normal retroplacental hypoechoic "clear zone" between placenta and myometrium • Thinning or complete absence of the myometrium overlying the placental bed (<1 mm is concerning) • Bulging of the placenta/uterine serosa into the bladder • Focal exophytic mass extending beyond the serosa (percreta)

No single sign is diagnostic in isolation; a validated ultrasound scoring system that combines multiple markers performs better than any single feature.

Color and 3D power Doppler findings

Color Doppler adds vascular information that grayscale alone cannot provide:

• Turbulent, high-velocity flow within lacunae — the hallmark "lake" pattern of chaotic, non-laminar flow • Hypervascularity of the uterovesical interface — a dense tangle of vessels bridging the myometrium and bladder wall • Bridging vessels crossing the myometrium-bladder interface, perpendicular to the myometrial surface, are highly specific for deep invasion • Loss of the normal continuous bladder wall echo line correlates with increased risk of bladder invasion (percreta)

3D power Doppler can further characterize the "inseparable" cotyledonal/intervillous circulation seen in confirmed PAS, though it is used mainly in research and complex referral settings.

Bridging vessels that cross perpendicular through the myometrium into the bladder wall on color Doppler are one of the most specific single findings for percreta and should prompt urology and interventional radiology consultation.

The role of MRI

MRI does not outperform high-quality ultrasound for anterior, easily visualized placentas, but it adds value when:

• The placenta is posterior, making sonographic assessment of the myometrium-bladder interface difficult • Maternal body habitus limits ultrasound acoustic windows • Parametrial or broad-ligament extension is suspected, which affects surgical approach • Preoperative surgical mapping is needed at a Center of Excellence

Key MRI features include dark intraplacental bands on T2-weighted imaging, uterine bulging, heterogeneous placental signal, and direct visualization of loss of the myometrial dark line. MRI is typically performed without gadolinium contrast in pregnancy, relying on T2-weighted sequences.

Imaging findings by PAS severity grade

Multidisciplinary Delivery Planning

Once PAS is suspected or confirmed, care shifts from routine obstetrics to coordinated, protocol-driven planning. Delivery at a Center of Excellence with an experienced multidisciplinary PAS team is consistently associated with lower blood loss, fewer urologic injuries, and lower maternal morbidity than unplanned or emergent surgery at a general hospital.

• PAS COE: Recommended delivery setting (Center of Excellence)
• 34–36 wk: Typical scheduled delivery (earlier if percreta/bleeding)
• 4–6 U: Units cross-matched (severe PAS) (PRBCs, plus FFP/platelets)
• 6–8: Core MDT services (MFM, anesthesia, urology, IR, blood bank, NICU)

Assembling the team

A PAS delivery plan is built by a standing multidisciplinary team rather than assembled ad hoc on the day of surgery. Core members typically include:

• Maternal-fetal medicine or PAS-experienced pelvic surgeon (primary operator) • Gynecologic oncology or advanced pelvic surgery backup for extensive dissection • Obstetric anesthesiology, prepared for rapid transfusion and possible general anesthesia conversion • Urology, for cystoscopy, ureteral stent placement, and bladder repair when percreta involves the bladder • Interventional radiology, for consideration of prophylactic balloon catheter placement or embolization capability • Blood bank / transfusion medicine, activating a massive transfusion protocol in advance • Neonatology / NICU, given the planned preterm delivery • Critical care, with a reserved ICU bed for postoperative recovery

Timing the delivery

Delivery is scheduled preterm, balancing the risk of spontaneous labor or hemorrhage against neonatal prematurity risk. Commonly used windows:

• Uncomplicated, stable accreta/increta: 36 0/7–37 0/7 weeks • Increta with bleeding episodes or higher-risk features: ~34–35 weeks • Percreta or recurrent antepartum hemorrhage: as early as 34 weeks, sometimes sooner if hemorrhage forces urgent delivery

Antenatal corticosteroids are given prior to scheduled preterm delivery to accelerate fetal lung maturity. Patients are counseled to present immediately for any bleeding, contractions, or rupture of membranes, since a substantial proportion of PAS patients deliver earlier than planned due to hemorrhage.

A written, team-reviewed delivery plan — incision site, ureteral stent decision, blood product targets, ICU bed reservation — should exist before the patient ever reaches the operating room.

Preoperative preparation

In addition to team assembly, preoperative steps commonly include:

• Preoperative cystoscopy with ureteral stent placement when bladder invasion is suspected, to help identify ureters intraoperatively and reduce injury risk • Type and cross-match for multiple units of packed red blood cells, with fresh frozen plasma, platelets, and cryoprecipitate available • Cell salvage (intraoperative blood recovery) setup where available • Preoperative iron repletion or erythropoietin in anemic patients when time allows • Counseling the patient and family about the planned hysterectomy, fertility loss, and realistic complication rates, including possible bowel or bladder injury and ICU admission

Cesarean Hysterectomy and Outcome

The cornerstone of modern PAS management is a scheduled cesarean hysterectomy in which the placenta is deliberately left undisturbed rather than manually removed. Attempting to separate an abnormally invasive placenta is the single most common cause of catastrophic, life-threatening hemorrhage in this condition.

• 2–5 L: EBL, planned PAS hysterectomy (vs 3–5 L+ if percreta)
• >5–10 L: EBL, attempted manual removal (high catastrophic-hemorrhage risk)
• ~40–50%: Maternal ICU admission (planned severe PAS cases)
• selected cases: Conservative (uterus-sparing) success (placenta left in situ, close follow-up)

Surgical strategy: incision away from the placenta

The hallmark technical principle of PAS surgery is avoiding the placenta entirely during hysterotomy. Preoperative imaging is used to map the placental margins, and the uterine incision — often a fundal or high vertical (classical) incision — is placed well away from the placental bed so the infant can be delivered without disturbing placental attachment.

After the infant is delivered and the cord is clamped, the hysterotomy is closed (or left partially open) and the surgeon proceeds directly to hysterectomy without attempting placental separation. The placenta is left in situ within the specimen.

Why the placenta is left in situ

Forcible or manual removal of an abnormally adherent placenta tears the underlying myometrium and its markedly hypervascular bed, producing sudden, torrential, difficult-to-control hemorrhage — historically the leading cause of PAS-related maternal death. Leaving the placenta attached and proceeding to hysterectomy removes the entire abnormal implantation site en bloc, controlling bleeding at its source (the uterine and, if needed, internal iliac vasculature) rather than fighting an open placental bed.

Adjuncts used during hysterectomy include prophylactic ureteral stents, staged ligation of the uterine and utero-ovarian vessels, interventional-radiology balloon occlusion of the internal iliac or common iliac arteries in select cases, and intraoperative cell salvage.

Attempted vaginal delivery or manual placental separation in an unrecognized PAS case can precipitate hemorrhage exceeding several liters within minutes — this is the scenario that structured prenatal screening is specifically designed to prevent.

Conservative management and long-term outcome

In carefully selected, hemodynamically stable patients who strongly desire future fertility, a uterus-conserving approach — leaving the placenta in situ without hysterectomy, with or without adjuvant methotrexate, and close outpatient surveillance for infection, delayed hemorrhage, and placental resorption — has been reported, primarily for less invasive (accreta) disease. This strategy carries meaningful risks of delayed hemorrhage, sepsis, and eventual hysterectomy, and is only appropriate at centers experienced in its follow-up protocol.

With planned, team-based cesarean hysterectomy at a Center of Excellence, most patients survive without organ injury, though composite severe maternal morbidity remains substantially higher than for routine cesarean delivery. Outcomes are consistently worse when PAS is diagnosed only at the time of delivery rather than prenatally, underscoring the value of the imaging pathway covered in Stage 2.