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 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.
Support if you have experienced a loss
Reading about miscarriage, stillbirth, or infant loss can be difficult. If you have experienced a loss, you do not have to use this calculator or read the statistics. You may find it more helpful to talk with your care team or a support organization.
In the United States, groups such as:
offer information and bereavement support. If you are elsewhere, your local health service or a pregnancy and baby loss charity may be able to guide you to support in your region.
Methodology and sources
- Gestational age at diagnosis (weeks and days): only the weeks value is used in the probability adjustment. The days value is displayed in results.
- Type: low-lying, marginal previa, or complete previa.
- Distance from the internal os (mm): used only when type is low-lying (to pick a distance band). It is still displayed for all types.
- Placenta position (advanced): anterior or posterior.
- Prior C-sections (advanced): 0 to 3.
- Multiple pregnancy (advanced): yes or no.
The calculator is a rules-based heuristic. It outputs three percentages: the estimated chance the placenta edge is clear of the cervix by 28 weeks, 32 weeks, and 36 weeks.
The calculator adjusts each baseline percentage using these rules:
- Placenta position: posterior adds 5 points. anterior subtracts 2 points.
- Gestational age at diagnosis (weeks only): if weeks < 20, add 5 points. if weeks >= 28, subtract 5 points. otherwise no change.
- Prior C-sections: if C-sections >= 2 and type is marginal or complete, subtract 5 points.
- Multiple pregnancy: subtract 3 points.
- Clamp and rounding: the result is rounded to a whole percent and clamped to 1% to 99%.
The calculator suggests a follow-up scan window based on the category and distance. It uses today's date and adds a fixed number of days. It does not compute the window based on gestational age dates.
- Complete previa: 14 days from today (window is start date through start date plus 4 days).
- Low-lying with distance 0 to 10 mm: 14 days from today (plus 4-day window).
- Marginal previa: 21 days from today (plus 4-day window).
- Low-lying with distance 11 to 20 mm: 21 days from today (plus 4-day window).
- Low-lying with distance greater than 20 mm: 35 days from today (plus 4-day window).
- Fallback: if none of the above matches, it uses 28 days from today.
The "Possible PAS" banner is shown when the type is marginal or complete (previa) and there is at least one prior C-section. The tool does not estimate a numeric PAS probability.
- Default note: if the placenta still covers or reaches the cervix late in pregnancy, delivery is usually planned in hospital and C-section is common.
- Exception: if type is low-lying and distance is greater than 20 mm, it shows a note that vaginal birth is often possible depending on local guidance and the individual situation.
- The probability output is a heuristic table with small adjustments. It is not a validated prediction model.
- Days at diagnosis are not used in the probability math (weeks only).
- The suggested re-scan window is based on "today" rather than gestational age scheduling (for example, "repeat at 32 weeks").
- Important clinical factors are not modeled (bleeding, ultrasound features of PAS, placental thickness, contractions, cervical length, IVF, prior uterine surgery other than C-section, and local protocols).
- Millimetre measurements vary by technique and bladder filling. A small measurement change can move a case between bands.