In one line
Placenta accreta spectrum (PAS) is iatrogenic disease — abnormal trophoblast invasion through the defective decidua of a previous caesarean scar — and the patient's life is decided antenatally, by diagnosing it before labour, delivering her in a centre with a multidisciplinary team and a massive-transfusion service at around 34–36 weeks, and performing a planned caesarean-hysterectomy that never disturbs the placenta; the complicated caesarean more broadly is the same discipline applied to dense adhesions, the deeply impacted or transverse-lying fetus, and the unplanned haemorrhage.
Mechanism & pathophysiology
The accreta spectrum is a defect of the maternal–fetal interface, not of the trophoblast itself. Normal implantation places anchoring villi against decidua basalis, a specialised endometrial layer that limits how deeply extravillous trophoblast invades and that later provides the cleavage plane through which the placenta separates. Where a previous caesarean has left a scar, the endometrium and superficial myometrium over that scar heal by fibrosis with absent or deficient decidua. A subsequent blastocyst implanting low and anteriorly — over the scar — meets no decidual barrier, so extravillous trophoblast and anchoring villi invade unchecked into and through the myometrium. The favoured model, set out in the ACOG/SMFM consensus, is precisely this: a defect of the endometrial–myometrial interface causes failed decidualisation in the area of a uterine scar, allowing abnormally deep anchoring villi and trophoblast infiltration. It is therefore a wound-healing disease, and the strongest single predictor is the number of previous uterine scars.
Depth of invasion defines the FIGO grades. In accreta (FIGO grade 1) villi adhere directly to myometrium without intervening decidua but do not invade it; in increta (grade 2) villi invade into the myometrium; in percreta (grade 3) they breach the serosa (3a) and may invade the bladder (3b) or other pelvic organs (3c). The clinical danger scales with depth because the abnormal placental bed recruits a florid, high-flow neovasculature — dilated subplacental and uterovesical vessels with disordered architecture — so that any attempt to separate the placenta opens vessels that cannot contract down, and exsanguinating haemorrhage follows within minutes. The histological reality at the time of surgery is that there is no separation plane to find: the placenta and myometrium are one continuous, engorged mass, which is why the entire operative strategy is built around not looking for a plane that does not exist. The depth is also not uniform — a placenta may be accreta over most of its bed and increta or focally percreta at one point, so the worst area dictates the plan.
The accuracy of depth assessment matters because it changes the operation, not just the label: a focal accreta over the scar in a woman who has completed her family is managed differently from a percreta invading the bladder dome, where urology must be scrubbed and the bladder dome may be deliberately resected with the specimen. The disordered uterovesical neovasculature of percreta is the single most dangerous anatomy in obstetric surgery — large, thin-walled, high-pressure vessels running between an invaded bladder and the uterus, with no surgical plane and no capacity to constrict — and underestimating it preoperatively is the commonest route to an uncontrolled loss of several litres in minutes.
Praevia is the co-conspirator, and the synergy with prior caesarean is the number to know. The terminology is distance-based: measured after 16 weeks, a low-lying placenta is one whose leading edge lies less than 20 mm from the internal os, and a placenta praevia is one that covers the os (the older grade I to IV classification is obsolete). A low-lying placenta is far more likely to overlie a scar, and praevia and accreta share the same risk factor. Silver's prospective cohort of women with praevia quantified the interaction exactly: the risk of accreta in a woman with praevia rises from 3% with no prior caesarean to 11%, 40%, 61% and 67% with one, two, three and four or more previous caesareans. The lesson is that praevia in a scarred uterus is accreta until imaging proves otherwise, and the third or fourth repeat caesarean with an anterior low placenta is the archetypal high-risk patient. The same biology underlies the caesarean scar ectopic pregnancy — implantation within the scar niche itself — which the 2026 RCOG guideline now treats in a dedicated section as an early-gestation precursor of both placenta praevia and the accreta spectrum, to be recognised and managed as such rather than as an ordinary low miscarriage.
Assessment
Risk stratification begins before any scan, from the obstetric history.
- Count the scars and place the placenta. The two variables that drive risk are the number of prior caesareans and an anterior placenta praevia or low-lying placenta overlying the scar. A single risk factor (one prior CS, posterior praevia) is low risk; their combination — an anterior praevia in a woman with two or more previous caesareans — is the high-probability accreta until disproved.
- The accreta spectrum is not exclusively a disease of praevia or previous caesarean (an emphasis strengthened in the 2026 RCOG guideline): any procedure that scars the endometrial–myometrial interface raises the risk — previous myomectomy entering the cavity, uterine curettage, hysteroscopic surgery, endometrial ablation, uterine artery embolisation, manual removal of placenta, Asherman treatment, and increasing maternal age and parity. PAS can occur without any prior surgery, but uncommonly.
Ultrasound is the first-line diagnostic test, and a focused greyscale plus colour-Doppler examination by an experienced operator is as accurate as MRI for most cases. The greyscale signs to seek are: loss of the normal hypoechoic retroplacental clear (myometrial) zone; multiple irregular vascular lacunae ("Swiss-cheese" or "moth-eaten" placenta), the single most useful sign; thinning or interruption of the hyperechoic uterine serosa–bladder wall interface; and focal exophytic placental tissue bulging beyond the serosa. Colour Doppler adds turbulent high-velocity lacunar flow, abnormal subplacental hypervascularity, and bridging vessels crossing the placenta–myometrium–bladder plane. The accreta workup is therefore done at the time of the routine anomaly scan whenever the placenta is low and anterior with a prior scar — it is a deliberate, directed examination, not an incidental finding.
MRI is the adjunct, not the screening tool. It earns its place where ultrasound is equivocal, where the placenta is posterior (poorly seen on transabdominal/transvaginal ultrasound), and where defining the topography and depth of invasion changes the surgical plan — parametrial extension, lateral or posterior bladder involvement, ureteric proximity. The MRI features of invasion are dark intraplacental bands on T2, uterine bulging, heterogeneous placental signal, focal myometrial interruption, and tenting of the bladder. MRI does not improve the basic accreta-versus-no-accreta decision over good ultrasound; it informs the operative decision in advanced disease.
- Measure the placental position with transvaginal ultrasound, and document the lower-segment anatomy. Transvaginal ultrasound is safe in praevia and defines the placental edge-to-os distance more accurately than the transabdominal view; the 2026 guideline also uses it to assess cervical length as part of the antenatal plan. Whether the placenta covers the os, the edge-to-os distance, and the position relative to the scar all shape the hysterotomy plan (the incision must avoid the placenta).
- Baseline investigations stage the host as well as the placenta: full blood count and ferritin (correct anaemia before the operating list, not on the table), group-and-save with antibody screen and crossmatched units arranged ahead of time, coagulation, U&E/creatinine, and an HIV test with confirmation of suppressive antiretroviral therapy — antenatal anaemia and limited blood-product availability are the SA-specific multipliers of accreta mortality, and both are correctable in clinic.
- Cystoscopy is considered where percreta with bladder invasion is suspected, to plan urological involvement.