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First Trimester Screening Risk Calculator

Educational tool for calculating chromosomal anomaly probabilities based on MoM values and maternal age. Includes reference data for NT and biochemistry.

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Category Medicine & Tests
DISCLAIMER: Educational use only. Not for medical diagnosis.
Valid range: 45-84mm
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About

This educational utility demonstrates how probability models estimate the risk of fetal chromosomal anomalies. Medical professionals use specific biochemical markers and ultrasound measurements to calculate a likelihood ratio. This ratio adjusts the background risk associated with maternal age. Accuracy depends on precise inputs for Crown Rump Length (CRL) and Nuchal Translucency (NT). The calculation relies on MoM (Multiples of the Median) to normalize data across different gestational ages. Small deviations in NT thickness can significantly alter the risk profile for conditions like Trisomy 21.

prenatal screening trisomy 21 medical calculator pregnancy health fetal medicine

Formulas

The final risk calculation combines the a priori risk with likelihood ratios derived from screening markers.

Risknew = Riskage × LRNT × LRbiochem

The Multiples of the Median (MoM) normalizes the measured value against the expected median for that gestational age.

MoM = Measured ValueMedian for Age

Reference Data

ParameterDefinitionTypical Median
PAPP-APregnancy-associated plasma protein A1.0 MoM
Free β-hCGHuman chorionic gonadotropin (beta subunit)1.0 MoM
NTNuchal Translucency thicknessf(CRL)
Background RiskA priori risk based on maternal age1:AgeFactor

Frequently Asked Questions

MoM stands for Multiples of the Median. It indicates how far a test result deviates from the average expected result for a specific gestational age. A value of 1.0 is exactly average.
Chromosomal anomalies occur more frequently as maternal age increases. This forms the background risk which the screening test then modifies based on blood work and ultrasound findings.
Nuchal Translucency measures the fluid at the back of the fetal neck. Increased thickness is often associated with a higher likelihood of chromosomal conditions.
No. This tool is strictly educational. It simulates the mathematical process used in screening but cannot provide a medical diagnosis or replace professional consultation.