Placenta Praevia, Placenta Praevia Accreta and Vasa Praevia: Diagnosis and Management

Guideline Structure

The original combined No. 27 guideline is no longer maintained as a standalone document on the RCOG website — visiting its URL now redirects to the two component guidelines. Candidates should think of "GTG 27" as an umbrella covering three related but distinct diagnoses (placenta praevia, placenta accreta spectrum, and vasa praevia), each with its own risk profile, screening trigger, and delivery plan, detailed fully in 27a and 27b.

Definitions and Classification

• Low-lying placenta: placental edge <20 mm from the internal cervical os, not covering it
• Placenta praevia: placenta partially or completely covers the internal cervical os
• Older "major/minor praevia" grading has been superseded by this descriptive, distance-based classification, which maps directly onto the mode-of-birth decision at the 36-week scan
• Placenta accreta spectrum (PAS): a spectrum of abnormally invasive placentation — accreta (adherent to myometrium), increta (invades myometrium), percreta (penetrates through myometrium/serosa, may involve bladder or bowel) — see GTG 27a for full staging and imaging criteria
• Vasa praevia: unprotected fetal vessels (not cushioned by placental tissue or umbilical cord) running through the membranes over or near the internal os — see GTG 27b

Screening and Diagnosis Pathway

• Placental location is assessed at the routine 20-week (18–20+6 weeks) anomaly scan
• A low-lying placenta or praevia found at this stage does not confirm the diagnosis: roughly 9 in 10 women with a low-lying placenta at 20 weeks no longer have one later in pregnancy, and only about 1 in 200 women overall have praevia at term
• If low-lying/praevia at the anomaly scan, a follow-up scan at 32 weeks is offered — transvaginal ultrasound (TVUS) is safe and more accurate than transabdominal scanning and is used to confirm placental edge distance from the os
• If still low-lying/praevia at 32 weeks, a further scan at 36 weeks is offered to finalise the diagnosis and plan mode of birth
• Cervical length may be measured at the 32-week scan as an adjunct to predict preterm birth/bleeding risk
• Placenta accreta spectrum is screened for opportunistically in at-risk pregnancies (praevia plus prior caesarean(s)) using greyscale/Doppler ultrasound, with MRI as an adjunct where invasion depth or posterior placentation makes ultrasound assessment difficult
• Vasa praevia is not part of routine national screening; targeted TVUS with colour Doppler is offered where risk factors are present (low-lying placenta, multiple pregnancy, IVF pregnancy, bilobed/succenturiate-lobe placenta, velamentous cord insertion)

High-Yield Exam Points

• GTG 27 itself is superseded — cite 27a (praevia/accreta) or 27b (vasa praevia) for management-level exam answers
• <20 mm from os = low-lying; covering os = praevia — know this cut-off precisely
• 9/10 low-lying placentas at 20 weeks resolve; only ~1/200 pregnancies have praevia at term
• TVUS is safe in suspected praevia and is the confirmatory test, not a contraindication
• Scan cascade to remember: 20 weeks (screen) → 32 weeks (rescan if abnormal) → 36 weeks (confirm and plan delivery)
• Placenta accreta spectrum and vasa praevia are separate diagnoses sharing overlapping risk factors (praevia, prior uterine surgery) but requiring distinct screening pathways and delivery plans

Source: RCOG Green-top Guideline No. 27 (overview — see GTG 27a for placenta praevia/accreta management detail, GTG 27b for vasa praevia detail) (GTG 27 (3rd edition) was published in 2011 and is now superseded/archival. In September 2018 RCOG split it into two standalone guidelines: GTG 27a (*Placenta Praevia and Placenta Accreta: Diagnosis and Management*) and GTG 27b (*Vasa Praevia: Diagnosis and Management*). GTG 27a is now in its 5th edition (last reviewed 30 June 2026, next review due 2029); GTG 27b dates from 2018. This overview page reflects the classification and pathway shared by both.)

Read the original on rcog.org.uk

Clinically reviewed by NHS obstetricians and gynaecologists · Source last updated GTG 27 (3rd edition) was published in 2011 and is now superseded/archival. In September 2018 RCOG split it into two standalone guidelines: GTG 27a (*Placenta Praevia and Placenta Accreta: Diagnosis and Management*) and GTG 27b (*Vasa Praevia: Diagnosis and Management*). GTG 27a is now in its 5th edition (last reviewed 30 June 2026, next review due 2029); GTG 27b dates from 2018. This overview page reflects the classification and pathway shared by both.

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