First Trimester Prenatal Screening Calculator (MoM)

Calculate Multiples of the Median (MoM) for PAPP-A and hCG levels. Risk assessment utility for Trisomy 21/18/13 screening.

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About

The First Trimester Combined Screening is a vital prenatal test performed between 11 and 14 weeks of pregnancy. It combines an ultrasound measurement (Nuchal Translucency) with two biochemical markers in the mother's blood: PAPP-A (Pregnancy-Associated Plasma Protein A) and free β-hCG (Human Chorionic Gonadotropin). However, raw blood values vary wildly between patients.

To make these values comparable, medical professionals convert them into MoM (Multiples of the Median). A MoM value represents how far a patient's result deviates from the "average" (median) for their gestational age. For example, a MoM of 1.00 is exactly average. Significantly low PAPP-A MoM or high hCG MoM can indicate an increased risk for chromosomal anomalies like Down Syndrome (Trisomy 21). This tool helps clinicians and patients understand these statistical deviations before consulting a genetic counselor.

prenatal screening trisomy risk mom calculator pregnancy health papp-a hcg

Formulas

The MoM value is a ratio of the patient's measured concentration to the expected median concentration for a healthy pregnancy at that specific gestational day.

MoM = ConcentrationpatientMediangestational_age

Corrected MoM values may also factor in weight, smoking status, and ethnicity:

MoMcorr = MoMraw ÷ (Wfactor × Sfactor)

Reference Data

MarkerNormal Range (MoM)Trend in Trisomy 21Trend in Trisomy 18/13
PAPP-A0.50 - 2.00Low (< 0.50)Very Low (< 0.30)
Free β-hCG0.50 - 2.00High (> 2.00)Very Low (< 0.30)
Nuchal Translucency< 1.50High (> 2.00)High (> 2.00)
Placental Function~1.00ReducedSeverely Reduced
Gestational Age11w - 13w6dN/AN/A

Frequently Asked Questions

A MoM of 0.5 means the level of that protein in your blood is exactly half of what is considered the median (average) for your gestational age. While values between 0.5 and 2.0 are generally considered normal, isolated low or high values are combined with other factors to assess risk.
No. This is a *screening* test, not a *diagnostic* test. A 'High Risk' result (e.g., 1 in 50) means that out of 50 women with exactly your results, 1 is carrying a baby with the condition. It warrants further testing like NIPT or Amniocentesis.
The levels of PAPP-A and hCG change rapidly during the first trimester. PAPP-A rises exponentially, while hCG drops. If the dating of the pregnancy is off by even a few days, the expected median changes, making the MoM calculation inaccurate.
Low PAPP-A (< 0.4 MoM) can be associated with placental issues later in pregnancy (like pre-eclampsia or growth restriction) even if chromosomes are normal. Your doctor may recommend extra growth scans in the third trimester.