Use this tool to estimate Placenta Accreta Spectrum (PAS) risk based on placenta position and prior caesareans. It returns a simple risk band and educational next steps.
How the estimate is built
- Core drivers: PAS risk rises when a placenta previa or low-lying placenta co-exists with prior caesarean section(s), particularly with an anterior placenta over the scar (ACOG Obstetric Care Consensus: Placenta Accreta Spectrum; RCOG Green-top Guideline 27a).
- Baseline bands: we start from an educational baseline mapped to previa status (none, low-lying, marginal, complete) and number of prior C-sections (0, 1, 2, 3+), reflecting stepwise increases reported in consensus guidance (FIGO PAS Consensus).
- Adjustments: small increments for anterior location and suspected scar overlap; small decrements for posterior placentas. IVF pregnancy and older maternal age add minor upward adjustments (contextual risk markers noted by societies; see sources below).
- Output: the final value is grouped into Low, Moderate, or High with a short plain-language context. Figures are rounded for readability.
Follow-up and planning (educational)
- Imaging: expert targeted ultrasound is first-line; consider MRI when ultrasound is indeterminate or for surgical planning (ACOG; SMFM Consult Series on PAS).
- Referral: moderate/high bands suggest discussion with a unit experienced in PAS care (FIGO Consensus).
- Delivery planning: when PAS remains suspected, plans usually include delivery in a hospital with blood products and senior multidisciplinary support (RCOG 27a).
Assumptions and limitations
- Terminology and thresholds for “low-lying,” “marginal,” and ultrasound signs vary by institution; we use simplified groupings for public understanding.
- Detailed imaging features (e.g., lacunae, loss of clear zone, bridging vessels) and intraoperative history can change risk substantially; this model does not replace expert review (ACOG).
- Other factors (number/location of uterine scars, placenta percreta clues, posterior variants) are summarised for context but not fully modeled.
- ACOG Obstetric Care Consensus: Placenta Accreta Spectrum — risk factors, imaging pathway, delivery planning.
- RCOG Green-top Guideline 27a — definitions (low-lying/previa), thresholds, and management recommendations.
- SMFM Consult Series: Placenta Accreta Spectrum — ultrasound vs MRI roles; referral and delivery planning.
- FIGO Consensus Guidelines on PAS — global consensus on diagnosis and multidisciplinary care.
- NHS: Placenta praevia — patient-facing overview of definitions, risks, and care pathways.
How to use this calculator
- Select your previa status: none, low-lying, marginal, or complete.
- Enter prior caesarean count.
- Choose placenta location: anterior or posterior.
- Optional: note IVF pregnancy, maternal age, and any report comment about scar overlap.
- Read the PAS risk band with short context and educational follow-up notes.
Educational only, not medical advice. Seek urgent care for heavy bleeding, severe pain, dizziness, or fainting.
Frequently asked questions
What is Placenta Accreta Spectrum (PAS)?
PAS describes abnormal placental attachment into or through the uterine wall. It ranges from accreta to increta and percreta.
Why do previa and prior caesareans raise risk?
A placenta over a uterine scar has a higher chance of abnormal attachment. Risk can rise with the number of prior caesareans, especially with anterior placenta over the scar.
Does placenta location matter?
Anterior placentas over a scar can carry higher risk than posterior in the same context. Location helps frame how closely to monitor.
What follow-up might be suggested?
Targeted ultrasound by an experienced sonographer and, in selected cases, MRI. Referral to a centre with PAS experience may be advised for higher-risk results.
Does IVF change PAS risk?
Some studies report a higher PAS risk with IVF pregnancies, especially with previa. The calculator captures IVF as optional context.
When should I seek urgent assessment?
Late-pregnancy bleeding or pain needs urgent assessment. If unsure, contact your care team or emergency services.