First Trimester Prenatal Screening Risk Calculator
Assess risk for Trisomy 21, 18, and 13 using Age, NT, and Biochemistry markers. Uses likelihood ratios based on Fetal Medicine Foundation algorithms.
About
Between 11 and 13 weeks of gestation, a combined screening test helps estimate the probability of chromosomal anomalies such as Trisomy 21 (Down Syndrome), Trisomy 18 (Edwards), and Trisomy 13 (Patau). This calculator combines Maternal Age Risk (Background Risk) with biophysical markers (Nuchal Translucency) and biochemical markers (Free β-hCG and PAPP-A) to generate an adjusted risk ratio.
Results are derived using the Likelihood Ratio (LR) method. MoM (Multiples of the Median) values normalize the hormone levels for maternal weight and ethnicity. A high Nuchal Translucency (NT) or low PAPP-A increases the LR, raising the calculated risk. This tool helps interpret complex lab reports but is not a diagnostic tool; high-risk results typically require confirmation via CVS or Amniocentesis.
Formulas
The calculation follows a sequential Bayesian approach:
Likelihood Ratios (LR) are derived from the distribution of markers in affected vs. unaffected pregnancies. For Biochemistry, values are converted to MoM:
In Trisomy 21 pregnancies, PAPP-A is typically 0.5 MoM and β-hCG is 2.0 MoM.
Reference Data
| Maternal Age | Background Risk (T21) | Background Risk (T18) | Background Risk (T13) |
|---|---|---|---|
| 20 | 1:1527 | 1:18000 | 1:42000 |
| 25 | 1:1350 | 1:16000 | 1:38000 |
| 30 | 1:900 | 1:11000 | 1:25000 |
| 35 | 1:350 | 1:4000 | 1:9000 |
| 40 | 1:85 | 1:1000 | 1:2500 |
| 45 | 1:35 | 1:400 | 1:1000 |