Educational draft · awaiting clinical reviewThe full textbook explains uncertainty but does not replace live national or local guidance, specialist advice, or current prescribing information.
Identify abnormal placental attachment early, arrange specialist surgical planning, and prevent catastrophic bleeding from unplanned placental separation at birth.
Placenta accreta spectrum is primarily a problem of attachment and distorted uterine anatomy. A normal placenta separates through the decidual interface after birth. When placentation develops in an area of deficient decidua and uterine scarring, villous tissue can attach abnormally to the myometrium. The surrounding lower segment may become thinned, dehiscent and highly vascular. Historical terms such as accreta, increta and percreta describe different appearances and depths, but antenatal risk assessment should not imply that imaging predicts histological severity perfectly. The clinically important question is whether separation and pelvic dissection are likely to cause major haemorrhage.
Previous caesarean birth combined with an anterior low placenta is the key screening scenario. Review operative history rather than merely recording parity: myomectomy, uterine perforation, repeated intrauterine procedures and a prior caesarean-scar pregnancy may change risk. RCOG 2026 recommends recognising high-risk patients from booking and assessing relevant ultrasound signs at the anomaly scan. NICE's detailed pathway includes expert greyscale and colour Doppler assessment around 28 weeks, no later than 29 weeks, when the 20-week scan shows low placentation in a scarred uterus. Suspicious findings already visible earlier should trigger specialist input without waiting for that later date.
A caesarean scar defect can contain deficient decidua and altered myometrium, creating an abnormal placental attachment site that lacks the usual separation plane.
Previous myometrial surgery or repeated endometrial injury may predispose to abnormal attachment, although the combination of caesarean scarring and low placentation is especially important.
Villous tissue attaches at an abnormal maternal interface instead of maintaining the usual plane that permits placental separation after delivery of the infant.
Growth of a scar-associated placenta can accompany marked lower-segment thinning, dehiscence and distortion of surrounding tissues, complicating interpretation of apparent invasive depth.
Attempts to detach abnormal attachment can open a large vascular bed without an effective contraction and separation mechanism, producing sudden severe haemorrhage.
An anterior placenta reaching the cervix in a patient with previous caesarean birth is a high-priority imaging and referral finding.
Irregular lacunae and vessels crossing expected tissue planes are concerning when combined with lower-segment scarring and altered placental interfaces.
Marked lower-segment bulging, extensive surface vessels or distortion of the bladder interface may reveal unsuspected accreta before placental delivery.
Resistance to expected placental separation should prompt a pause and senior assessment rather than escalating force or repeated blind instrumentation.
A low placenta increases haemorrhage risk even with normal attachment, so imaging must distinguish the positional finding from the probability of abnormal adherence.
Failure of placental expulsion can occur without accreta; the circumstances and operative findings must be assessed before assuming either benign retention or pathological attachment.
A thin or deficient uterine scar can create a dramatic anatomical appearance; expert assessment distinguishes tissue remodelling from the pattern of abnormal placental attachment.
Placental disruption or difficult dissection may require large-volume transfusion and haemostatic surgery, including hysterectomy when preservation cannot safely control the bleeding.
Distorted vascular anatomy and loss of normal tissue planes increase the risk of bladder, ureteric and other pelvic injury during complex surgery.
When placenta remains in situ, secondary haemorrhage and infection can develop after initial recovery, requiring access to urgent review and sometimes further surgery.
The extent and location of abnormal attachment, uterine thinning and vascular distortion determine surgical difficulty more directly than an unqualified antenatal subtype.
A request should state what ultrasound could not resolve and how the answer will change care, rather than treating MRI as a mandatory diagnostic gate.
Conservative care carries delayed haemorrhage and infection risks and requires reliable prolonged follow-up; it is not equivalent to discharge without a treatment plan.
Routine prophylactic ureteric stents or interventional-radiology procedures are not supported for every case in RCOG 2026; the specialist team selects them for specific circumstances.
It should not be prescribed to accelerate resolution of placenta deliberately left in situ because benefit is unproven and maternal toxicity is possible.
The June 2026 guideline's key recommendation and section 10.2 table give different PAS delivery windows. Resolve this with the specialist accreta service and current RCOG advice rather than using either date as an automatic rule.
Attempting forceful manual removal when the placenta fails to separate in a patient with a strong accreta risk profile.
Waiting for an MRI result before referring a clearly suspicious ultrasound to a specialist centre.
Applying older routine delivery timing without discussing the revised 2026 recommendation and the patient's actual bleeding or preterm-birth risk.
Equating a uterine-preserving procedure with the end of maternal risk and omitting emergency follow-up arrangements.
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The anomaly scan shows an anterior low-lying placenta in a patient with two previous caesarean births. Expert Doppler later suggests accreta spectrum. What is the most appropriate next organisational step?
National guidance is shown before implementation-dependent detail. Typical adult dose examples remain subject to patient factors, contraindications and the live BNF or specialist protocol. Source check completed 7 Sept 2026; clinical approval remains outstanding.
Editorial contact: Doctor’s Passport editorial team. Source-check date is separate from clinical approval. Report a content problem · Content changes
Ultrasound findings can include abnormal placental lacunae, loss of the expected interface, myometrial thinning, placental bulging and bridging vessels. They require interpretation by an experienced operator in the context of scar location and placental position. MRI may add information when a specific anatomical question remains unresolved, particularly for surgical planning, but is not a routine prerequisite for diagnosis or referral. A reassuring scan lowers risk only to the degree supported by image quality and expertise; it cannot erase a strong clinical history.
Care is planned around the possibility of transfusion, hysterectomy, urinary-tract injury and critical-care admission. Discuss these possibilities before labour, including fertility wishes, anaesthetic choices, blood-product preferences and the infant's expected neonatal needs. The key recommendations in RCOG's 2026 guideline give 36+1–37+0 weeks when antenatal bleeding and other preterm-birth risks are absent. However, its section 10.2 table gives 35+0–36+6 weeks. This internal discrepancy requires explicit specialist reconciliation with the latest RCOG advice when agreeing the birth date; symptoms can require earlier delivery. The operating team selects a uterine incision that avoids the placenta and decides between hysterectomy and carefully selected uterine-preserving approaches. Conservative treatment requires prolonged access to review and emergency care because the risk continues after the infant is born.
Ensure blood products are physically available, monitor calcium and temperature during transfusion, and coordinate thromboprophylaxis timing with haemostasis and neuraxial anaesthesia.