Placenta accreta spectrum (PAS) is a group of pathologies describing abnormal adherence or invasion of the placenta into the myometrium. Three grades exist: accreta (absent decidua basalis, villi adherent to myometrial surface), increta (villi invade the myometrium), percreta (villi reach the serosa and adjacent organs — especially the bladder). Incidence is increasing parallel to rising cesarean rates (current estimate ~1/533 deliveries). The most important risk factor is the combination of placenta previa + prior cesarean. Prenatal diagnosis is critical in preventing massive obstetric hemorrhage — multidisciplinary surgical planning reduces mortality from 40% to <5%. Ultrasound is the primary diagnostic tool; MRI is used complementarily, especially for posterior placenta and invasion depth assessment.
Age Range
20-45
Peak Age
35
Gender
Female predominant
Prevalence
Rare
The fundamental mechanism of PAS is absent or defective decidua basalis layer. In normal pregnancy, the decidua basalis creates a natural barrier preventing trophoblastic invasion from reaching the myometrium. Trauma from prior cesarean scar, curettage, or uterine surgery damages this decidual layer → fibrosis and inadequate decidualization at the scar allow trophoblasts direct contact with myometrium → abnormal invasion. On ultrasound, this invasion manifests as myometrial thinning, placental lacunae (dilated vascular spaces in intervillous spaces), and loss of retroplacental 'clear space'. Lacunae contain turbulent blood flow — showing high-velocity jet streams on Doppler. On MRI, invasive trophoblastic tissue creates T2 hypointense bands within the myometrium — representing a mixture of fibrosis, hemosiderin, and desmoplastic reaction. In percreta, trophoblastic tissue extends beyond the serosa reaching the bladder wall → neovascularization and enhancement at the utero-vesical surface.
Combination of multiple irregular vascular lacunae ('Swiss cheese') within the placenta and loss of retroplacental clear space — the most reliable ultrasonographic finding combination for PAS diagnosis. Turbulent flow detection in lacunae on Doppler increases diagnostic value.
Irregular, multiple vascular spaces (lacunae) are seen within the placenta — creating a 'Swiss cheese' or 'moth-eaten' appearance. Lacunae represent abnormally dilated vascular areas in intervillous spaces. Increasing number correlates with higher accreta probability. Can be detected as early as the first trimester but most reliable assessment is in second-third trimester.
Report Sentence
Multiple irregular vascular lacunae are seen within the placenta with 'Swiss cheese' appearance, consistent with placenta accreta spectrum.
In normal pregnancy, a hypoechoic retroplacental 'clear space' (decidual zone and dilated myometrial veins) is seen between the placenta and myometrium. In PAS, this space is lost — the placenta is directly adherent to the myometrium. Detection of this loss, especially at the bladder-uterine interface, has high sensitivity for accreta diagnosis. However, in normal pregnancies, the clear space may not be clearly visible in up to 50% of cases — so it is not diagnostic alone.
Report Sentence
Loss of retroplacental 'clear space' is noted in the lower uterine segment, suspicious for placenta accreta spectrum.
High-velocity turbulent flow is seen within lacunae on color Doppler — mosaic pattern (color aliasing). Vascular bridges between lacunae, vessels traversing the myometrium, and abnormal vascularity extending to the cervix may be seen. Pulse-wave Doppler demonstrates low-resistance, high-velocity arterial flow. These findings increase the sensitivity of accreta diagnosis.
Report Sentence
High-velocity turbulent flow (mosaic pattern) is demonstrated within placental lacunae on color Doppler, consistent with placenta accreta spectrum.
Irregular hypointense bands are seen within the placenta on T2-weighted MRI. These bands represent fibrosis, hemosiderin deposition, and desmoplastic reaction created by invasive trophoblastic tissue in the myometrium. Band thickness and extent correlate with invasion depth. Additionally, myometrial thinning and focal myometrial loss are clearly evaluated on T2 sequences.
Report Sentence
Irregular hypointense bands are seen within the placenta on T2-weighted MRI, suggesting invasive trophoblastic tissue consistent with placenta accreta spectrum.
On T2-weighted MRI, the myometrium is extremely thinned (<1 mm) or focally lost at the placenta-bladder interface. Normal myometrium appears as a three-layered structure with intermediate T2 signal. In PAS, invasive trophoblastic tissue erodes the myometrium → focal discontinuity. In percreta, the myometrium is completely lost and trophoblastic tissue extends beyond the serosa to the bladder wall — uterine contour is disrupted with focal bulging into the bladder.
Report Sentence
Myometrial thinning/focal loss at the placenta-bladder interface is noted, suggesting the invasive form of placenta accreta spectrum (increta/percreta).
On contrast-enhanced MRI, prominent vascular structures and early enhancement are seen at the utero-vesical interface — representing neovascularization. This finding is particularly seen in percreta and is critical for surgical planning. Focal enhancement in the bladder wall is evidence of serosal invasion. Early arterial enhancement on dynamic contrast sequences highlights suspicious areas.
Report Sentence
Neovascularization and early enhancement at the utero-vesical interface are seen on contrast-enhanced MRI, consistent with percreta.
Criteria
Villi adherent to myometrial surface but not invading — decidua basalis absent
Distinct Features
Most common form (75-80%), myometrial thickness preserved but thinned, clear space loss, placenta does not separate at surgery
Criteria
Villi invade into the myometrium but do not reach the serosa
Distinct Features
Prominent myometrial thinning, T2 hypointense bands, focal myometrial loss, high surgical complication risk
Criteria
Villi extend beyond the serosa invading adjacent organs (bladder, bowel, parametrium)
Distinct Features
Complete myometrial loss, bladder bulging, utero-vesical neovascularization, bladder wall enhancement on MRI, highest mortality risk, hysterectomy frequently required
Distinguishing Feature
In isolated previa, myometrial thickness is preserved without lacunae; PAS shows lacunae + clear space loss + myometrial thinning
Distinguishing Feature
In abruption, retroplacental hematoma indicates placental separation; in PAS, the placenta invades the myometrium and cannot separate
Distinguishing Feature
Retained products show postpartum endometrial thickening and vascularity; PAS is diagnosed prenatally and requires surgical planning
Urgency
emergentManagement
surgicalBiopsy
Not NeededFollow-up
specialist-referralPAS is one of the most important causes of massive obstetric hemorrhage — prenatal diagnosis is life-saving. Planned cesarean hysterectomy (34-36 weeks) with a multidisciplinary team (maternal-fetal medicine, interventional radiology, anesthesia, urology, neonatology) is the standard approach. Intraoperative balloon occlusion or embolization can reduce hemorrhage. Conservative management (leaving placenta in situ with methotrexate) is attempted in selected cases but carries risk of infection and delayed hemorrhage.
PAS is a life-threatening obstetric emergency. Multidisciplinary planning is mandatory due to massive hemorrhage risk. Cesarean hysterectomy is frequently required in percreta. Prenatal diagnosis dramatically improves outcomes. After diagnosis, referral to a tertiary center, blood bank preparation, and experienced team planning are critical.