What this calculator does
This risk of Down syndrome calculator estimates the probability of a pregnancy being affected by trisomy 21 (Down syndrome) using maternal age and optional first-trimester screening markers. It is designed as an educational aid so you can better understand how screening factors can increase or decrease estimated risk.
The estimate is calculated from a baseline age-related risk and then adjusted with likelihood ratios from optional findings such as NT thickness, PAPP-A, free β-hCG, nasal bone status, and prior history. If you only enter age, the output reflects age-based background risk only.
How the estimate is calculated
1) Start with age-related baseline risk
Maternal age is the strongest baseline screening factor. The calculator uses an age-risk curve (interpolated between common reference points) to estimate the starting probability.
2) Convert probability to odds
Screening tests are typically combined using odds. The baseline probability is converted into prior odds: prior odds = p / (1 - p).
3) Apply marker likelihood ratios
Each optional marker contributes a likelihood ratio (LR). The calculator multiplies these together: posterior odds = prior odds × LRNT × LRPAPP-A × LRβ-hCG × LRnasal bone × LRhistory.
4) Convert back to probability
The final risk is shown as both a percentage and “1 in N.” This makes interpretation easier when discussing next steps with your clinician.
How to interpret your result
- Lower risk: typically less than 1 in 1000.
- Intermediate risk: around 1 in 1000 to 1 in 250.
- Increased risk: higher than 1 in 250.
These bands are only practical screening categories. Different hospitals, countries, or test providers may use different cutoffs.
What to do after calculating
If the estimate is low
A low estimate is reassuring but does not eliminate risk completely. Continue routine prenatal care and discuss whether additional screening is recommended based on your personal history.
If the estimate is intermediate or increased
Consider discussing next-step options such as cell-free DNA (NIPT), detailed ultrasound, chorionic villus sampling (CVS), or amniocentesis. Diagnostic tests can confirm or exclude chromosomal conditions more definitively.
Important limitations
- This tool is a simplified model and does not replace validated lab-specific risk software.
- Population-level statistics do not predict an individual outcome with certainty.
- Input quality matters; incorrect or incomplete values can materially change results.
- Only a diagnostic test can confirm Down syndrome.
Frequently asked questions
Is this a diagnosis?
No. This calculator provides a screening estimate only.
Can I use this without blood test values?
Yes. If optional markers are blank, you will get age-based risk alone.
Why does “1 in N” matter?
Many clinicians communicate prenatal screening in this format. For example, 1 in 500 means one affected pregnancy expected per 500 similar pregnancies.
Should I make decisions from this calculator alone?
No. Use this as a learning tool and confirm plans with an obstetrician, maternal-fetal medicine specialist, or genetic counselor.