Use this tool to interpret a low-lying placenta or placenta previa by gestational age, type, and distance from the cervix. It returns an educational resolution probability and a suggested re-scan window.
How this estimate works
- Definitions: “Complete previa” covers the internal os (0 mm), “marginal previa” reaches the os, and “low-lying” is close but not reaching, often within ~20 mm. See definitions and care pathways in RCOG Green-top 27a and the NHS overview.
- Resolution model: we estimate the chance that the placental edge will clear the cervix by 28, 32, and 36 weeks using the reported type and measured os distance, then modestly adjust for gestational age at diagnosis, reflecting that earlier findings have longer to move with uterine growth (RCOG 27a).
- Distance bands: 0 mm (complete), reaching the os (marginal), and low-lying bands ≤10 mm, 11–20 mm, and >20 mm. Greater distance is associated with higher likelihood of vaginal birth and spontaneous “clearance” later in pregnancy (Fetal Medicine Foundation).
- Gestational age: earlier diagnosis generally increases resolution probability as the lower uterine segment develops; later diagnosis reduces it (summarised in SMFM guidance on placenta previa and RCOG 27a).
- Placenta position: some series report higher clearance when the placenta is posterior, though findings are mixed. We apply only a small, educational nudge and advise following local protocols (FMF).
- Other factors: multiple pregnancy slightly lowers resolution chance; a prior caesarean can reduce clearance for true previa because of scarring near the lower segment (see ACOG: Placenta Accreta Spectrum).
- Planning note: if the placenta still reaches or covers the cervix late in pregnancy, care is usually hospital-based and caesarean birth is common; if the edge is clearly >20 mm from the os, a vaginal birth may be possible under unit policy (NHS; RCOG 27a).
Re-scan timing logic
- Higher concern: complete or marginal previa, or distance ≤10 mm → suggest about 2 weeks.
- Intermediate: distance 11–20 mm → suggest about 3 weeks.
- Lower concern: distance >20 mm and low-lying only → suggest about 5 weeks.
- These windows reflect common follow-up practice in the guidelines and assume no new bleeding; earlier review is sensible if symptoms change (SMFM; RCOG 27a).
Possible PAS (accreta spectrum) flag
The banner appears when there is previa (marginal or complete) and at least one prior caesarean, a combination that increases placenta accreta spectrum risk. This is an educational prompt only; diagnosis relies on targeted ultrasound or MRI when indicated (ACOG Obstetric Care Consensus; RCOG 27a).
Why placentas “move”
The placenta does not migrate like a separate organ. As the uterus enlarges and the lower segment stretches, the placental edge can end up further from the cervix even though implantation is fixed on the uterine wall (explained in Fetal Medicine Foundation and NHS materials).
Limitations
- Thresholds for “low-lying,” re-scan timing, and delivery planning vary by hospital and country.
- Measurements can vary by technique and fetal position; a few millimetres can change the category.
- Other factors not modeled here can change management, such as active bleeding, fetal growth, and suspected scar invasion.
- RCOG Green-top Guideline 27a: Placenta praevia and placenta accreta – definitions, follow-up, delivery planning, and PAS risk context.
- SMFM guidance on placenta previa: diagnosis and management – imaging, timing of reassessment, and counselling.
- ACOG Obstetric Care Consensus: Placenta Accreta Spectrum – PAS risk sharply increases with previa plus prior caesarean; evaluation pathways.
- Fetal Medicine Foundation: Placenta previa – distance from internal os, natural history, and counselling notes.
- NHS: Placenta praevia – patient-facing overview of symptoms, monitoring, and birth planning.
How to use this calculator
- Enter the gestational age at diagnosis (weeks and days).
- Enter the distance from the internal os in millimetres (from the report).
- Select the type: low-lying, marginal, or complete previa.
- Optional: add placental side (anterior/posterior), prior caesarean count, and multiparity.
- Read the estimated chance of resolution by 28, 32, and 36 weeks, plus a suggested re-scan window.
Educational only. Not medical advice. Seek urgent care for heavy bleeding, pain, dizziness, or fainting.
Frequently asked questions
What does “low-lying placenta” mean?
It means the placenta is close to the cervix. Reports often define low-lying by a measured distance from the internal os, while “previa” means overlapping the os.
Why do some previas resolve?
The placenta does not move, but the lower uterus grows and stretches as pregnancy advances. This can increase the distance between the placental edge and the cervix.
Does anterior vs posterior matter?
Placental side can influence resolution likelihood and imaging. The calculator accepts anterior or posterior as optional context; your result focuses on GA, type, and distance.
How is re-scan timing decided?
Many services re-scan based on the measured distance and type, often around 28–32 weeks, then again if still low. Your result suggests a window used by many clinics.
What is PAS (accreta spectrum) risk?
PAS risk can be higher when placenta previa occurs with prior caesarean scars. If previa and ≥1 prior C-section are present, consider reviewing the PAS Risk Calculator.
When should I seek urgent care?
Seek urgent care for heavy bleeding, severe pain, or fainting. If unsure, contact your care team or emergency services.