About

First-trimester screening combines maternal age, fetal nuchal translucency (NT) thickness, and maternal serum biochemistry to assess the risk of chromosomal anomalies. This tool estimates the risk for Trisomy 21 (Down Syndrome), Trisomy 18 (Edwards Syndrome), and Trisomy 13 (Patau Syndrome). The core of the calculation involves converting raw biometric values into Multiples of the Median (MoM), which normalizes data across different gestational ages. This standardization allows for accurate statistical comparison against population baselines.

Precision in measuring Crown-Rump Length (CRL) is critical, as median levels of PAPP-A and free β-hCG change rapidly during weeks 11 to 13. A discrepancy of just a few millimeters in dating can significantly alter the MoM value and the subsequent risk ratio. This calculator uses Fetal Medicine Foundation (FMF) derived regression curves for median estimations.

Formulas

The risk calculation proceeds in two steps. First, raw values are converted to MoM. Second, the background risk (from maternal age) is multiplied by Likelihood Ratios (LR) derived from the MoM values.

MoM = Measured ValueMedian for CRL

The Adjusted Risk is defined as:

Risk_adj = Risk_age × LR_NT × LR_biochem

Median regression models follow the exponential form:

Median = 10^{(a - b × CRL)}

Reference Data

Marker	Typical Trend (Trisomy 21)	Typical Trend (Trisomy 18/13)	Median Ref (12 Weeks)
Nuchal Translucency (NT)	Increased (> 2.0 MoM)	Increased	~1.5 mm
Free β-hCG	Increased (2.0 MoM)	Decreased (0.3 MoM)	~40 IU/L
PAPP-A	Decreased (0.5 MoM)	Decreased (0.2 MoM)	~3.0 IU/L
Maternal Age	Risk increases with age	Risk increases with age	N/A

Frequently Asked Questions

MoM stands for "Multiples of the Median". It represents how far a measured value deviates from the expected average (median) for a pregnancy of the exact same gestation. A MoM of 1.0 is exactly average. A MoM of 2.0 means the value is twice as high as expected.

The expected levels of PAPP-A and beta-hCG change daily during the first trimester. CRL is the most accurate method to date the pregnancy (between 45mm and 84mm), ensuring the measured biomarkers are compared against the correct reference standard.

No. A screening test gives a probability, not a diagnosis. A result of 1:50 means that out of 50 women with identical results, 1 will carry a fetus with the condition and 49 will not. High-risk results are typically followed by diagnostic testing (CVS or Amniocentesis).

This tool uses general population parameters. It does not account for factors like maternal weight, smoking status, ethnicity, or IVF conception, which professional clinical software corrects for to refine the MoM.