Suspected placenta accreta spectrum

RCOG GTG 27a; RANZCOG C-Obs 20

Ultrasound is the first-line test for placenta accreta spectrum and, in expert hands, performs comparably to MRI. MRI complements it where the placenta is posterior, where invasion depth or lateral extension is uncertain, or where parametrial involvement is suspected.

A woman with a low-lying placenta or placenta praevia and previous caesarean section or other uterine surgery, usually identified in the second trimester and reassessed in the third.

Referenced decision support — confirm against your local protocol.

Decision support, not a directive. Protocols and timings shown are typical published ones — your local protocol takes precedence, and the vetting radiologist decides.

The request in front of you

Everything is optional. Leave a field alone and the answer assumes nothing — the verdict updates as you go.

Study requested

What the referrer actually asked for. It is evidence of intent, not a constraint on the right answer.

Contrast as written

What the request form says, not what it should say.

Age

Pick a band, or type an exact age if it matters.

years

Pregnancy status

Renal risk factors

The question a vetter can actually answer from the request. An explicit “none known” is a real answer, and it removes checks rather than deferring them.

Previous contrast reaction

The pathway — tap anything already done

Marking a study complete moves the answer on. A patient arrives partway through a pathway far more often than at the start of one.

Accept as requested

Obstetric ultrasound — growth and wellbeing

Obstetric ultrasound

What we'd amend, and why

Greyscale and colour Doppler ultrasound demonstrates the features that define the spectrum — loss of the clear zone, myometrial thinning, placental lacunae, bladder wall interruption and bridging vessels — and it does so with accuracy comparable to MRI when performed by an experienced operator. It is available, repeatable through the pregnancy, and carries no risk to the fetus. Two technique points decide whether it answers the question. The bladder must be moderately full: it is the acoustic window through which the bladder–uterine interface, the loss of the clear zone and the bridging vessels are seen at all, and an empty bladder removes the finding rather than excluding it. And the examination is transabdominal AND transvaginal — transvaginal scanning is safe in praevia, and it is the only reliable way to measure the placental edge against the internal os and to assess the lower segment and cervix, which is where the invasion that matters sits.

Nothing needs resolving before this goes ahead. Routine checks below are owned downstream.

How was this decided?

pathwayany — Pregnant patients

Decision support only. Local protocol takes precedence.

Worth asking the referrer (1)

None of these hold the request up. They sharpen the protocol or the plan that follows.

How many previous caesarean sections or other uterine operations, and does the placenta overlie the scar?
It sets the pre-test probability and directs the detailed sonographic assessment of the bladder–uterine interface.

Pathways

Big forks are separate pathways; the first whose conditions match is the one used.

Pregnant patients

Matches your inputsDefault

Role	Study & protocol	Why this answers the question
First line	Obstetric ultrasound Obstetric ultrasound — growth and wellbeing	Greyscale and colour Doppler ultrasound demonstrates the features that define the spectrum — loss of the clear zone, myometrial thinning, placental lacunae, bladder wall interruption and bridging vessels — and it does so with accuracy comparable to MRI when performed by an experienced operator. It is available, repeatable through the pregnancy, and carries no risk to the fetus. Two technique points decide whether it answers the question. The bladder must be moderately full: it is the acoustic window through which the bladder–uterine interface, the loss of the clear zone and the bridging vessels are seen at all, and an empty bladder removes the finding rather than excluding it. And the examination is transabdominal AND transvaginal — transvaginal scanning is safe in praevia, and it is the only reliable way to measure the placental edge against the internal os and to assess the lower segment and cervix, which is where the invasion that matters sits.
Second line	MRI abdomen and pelvis MRI abdomen/pelvis — non-contrast rapid protocol	MRI complements ultrasound rather than replacing it. Its value is in the posterior placenta, which ultrasound reaches poorly, and in defining depth of invasion and lateral or parametrial extension — the findings that decide whether the operation is a caesarean hysterectomy and whether ureteric stents and interventional radiology are needed. The protocol is specific and a general obstetric MRI will not do it: T2-weighted single-shot sequences in three planes ORIENTED TO THE UTERUS rather than to the mother — sagittal, and axial and coronal planes angled to the placental bed and the uterine long axis — because the intraplacental dark bands, the bulge, the myometrial thinning and the bladder-wall tenting are all interface findings that a plane cutting obliquely across the interface destroys. A moderately full bladder is part of the protocol for the same reason it is on the ultrasound: it separates the bladder wall from the lower segment and makes the vesicouterine interface assessable. Balanced steady-state sequences add the vascular anatomy and T1 shows haemorrhage; the mother is imaged left-lateral or semi-decubitus to avoid aortocaval compression. Timing matters — around 28 to 32 weeks is the usual window, late enough for the findings to be developed and early enough for the delivery plan to be made. Gadolinium is not required and is avoided in pregnancy.

A negative MRI does not overturn convincing ultrasound findings, and vice versa. Where the two disagree, the delivery plan should follow the higher level of suspicion.
The output of this pathway is a date and a building. Where the spectrum is suspected, delivery is planned in a specialist centre with a multidisciplinary team, blood products and interventional radiology available, typically in the late-preterm window of about 35 to 37 weeks rather than at term — so a report issued after that decision has had to be made has answered the question too late to be useful.

Pitfalls

Assessing the bladder–uterine interface through an empty bladder, which removes the acoustic window that makes the interface visible.
Reporting placental position without addressing invasion in a woman with praevia and a previous caesarean — position alone does not answer the question that was asked.
Treating MRI as the arbiter. Both modalities are operator-dependent and have similar accuracy; discordance is a reason for multidisciplinary discussion, not for the second test to win.
Leaving the diagnosis to the third trimester when the delivery plan and the hospital of birth needed deciding weeks earlier.
Booking a general obstetric or abdominal MRI protocol. Planes angled to the mother rather than to the uterus cut obliquely across the very interface the study exists to assess, and the dark bands, the bulge and the bladder-wall tenting are lost — the study then has to be repeated at a gestation when there is no longer time to repeat it.
Scanning with an empty bladder on either modality, and then reporting the vesicouterine interface as intact. The window has been removed; the interface has not been assessed.
Omitting the transvaginal assessment because the placenta is praevia. Transvaginal scanning is safe in praevia and is the measurement the delivery plan is built on; declining it on safety grounds is a misconception, not a precaution.

Priors — what to pull first

Retrieve the earlier scans in this pregnancy: a placenta that has migrated away from the os removes the question, and one that has not is the whole point.
Operative notes from previous caesarean sections, particularly any recorded difficulty with the lower segment, inform the level of suspicion.

What makes a good request

The number of previous caesarean sections and whether the placenta overlies the scar are the two facts that generate the suspicion, so both belong on the request.
The purpose of imaging is planning: which hospital, which surgical team, what blood products, and what date. A report that does not address those is incomplete.
Confirmation should sit with an operator experienced in placenta accreta spectrum rather than with whoever is on the general list.

Scoring this once it is done

The classification and risk tools this question ends in.

Placenta Accreta Spectrum (PAS) Ultrasound/MRI Scoring Ultrasound Gestational Age & EDD

How these studies are acquired

Contrast, phases and timing for every study on the pathways above.

Obstetric ultrasound — growth and wellbeing MRI abdomen/pelvis — non-contrast rapid protocol

Confirm locally

Timings, contrast volumes and rates above are typical published values. Your department's protocol, scanner and patient population decide the actual numbers.
Safety thresholds and premedication policy follow local policy where it differs from the cited guidance.

References

Implemented from the cited published sources. Educational and workflow support only; confirm against current guidelines and local policy before clinical use.