First and second year — the floor first, then every step
In a woman with a previous caesarean and a low anterior placenta, look at the placenta–myometrium–bladder interface: lacunae, loss of the clear zone, myometrial thinning, bladder wall interruption and bridging vessels.
Orient first
The strongest risk is placenta praevia over a caesarean scar; the risk rises with the number of prior caesareans.
MRI has no equivalent of the ultrasound clear zone; on ultrasound it is the normal hypoechoic line between placenta and myometrium, and its loss is a sign — but it can be absent normally in the lower segment.
MRI is a problem-solver (posterior placenta, depth of invasion, parametrial extension) after ultrasound, not a replacement.
Acquire the study
Transabdominal and transvaginal views of the lower uterine segment with a moderately full bladder; colour Doppler at the uterovesical interface.
The manoeuvre
Placental lacunae: irregular vascular spaces with turbulent flow.
Loss of the retroplacental clear zone and myometrial thinning over the placenta (measure).
Bladder wall interruption or irregularity; placental bulge into the bladder.
Colour Doppler: bridging vessels across the uterovesical space and subplacental hypervascularity.
Placental position relative to the internal os and the scar.
What confirms it
Several concordant signs (not one) in a woman with risk factors.
What licenses you to exclude it
Absence of signs in a well-performed study lowers the likelihood, but a low-lying placenta over a scar still needs planned delivery in an experienced centre.
The classic misread
Calling a thin lower segment myometrium invasion on its own — it is normally thin late in pregnancy.
Reference values
Each value carries the caveat that keeps it from being misused. Normal limits and diagnostic criteria are kept apart on purpose: a disease cut-off read as a normal range is the more dangerous mistake.
loss of the retroplacental clear zone, myometrial thinning, placental lacunae, bridging vessels; on MRI, abnormal vascularity and interruption of the low-T2 interface. A praevia in a scarred uterus is the high-risk setting. Individual signs are non-specific — report the constellation. No invasion-depth millimetre is shipped
A previa in a scarred uterus is the high-risk setting. Individual signs are non-specific — report the CONSTELLATION and do not invent an invasion-depth millimetre. Versioned criterion — verify against the current edition before clinical use.
USG · MRI
Placenta (previa type) · Low-lying versus praevia — the registered os distance
above 20 mm from the internal os is normally sited; 20 mm or less is low-lying; overlying the os is praevia — the same figures as the registered placenta–os entry. Transvaginal, empty bladder. Most mid-trimester low-lying placentas resolve by term, so the gestation at assessment is part of the report
Do not invent a new millimetre hinge. Quote the registered distance and the current national table. A third-trimester transvaginal measurement is the reference. Versioned criterion — verify against the current edition before clinical use.
USG
See it on real cases
Direct links to Radiopaedia — the reference article and worked cases with their images. Each opens on Radiopaedia.