Why use it

Allows for the accurate use of first-trimester combined screening tools in twin pregnancies. Without correction, virtually all twin pregnancies would screen positive.

About

Maternal serum levels of PAPP-A and free beta-hCG are naturally higher in multiple gestations due to increased placental mass. If these Multiples of the Median (MoM) are not corrected for chorionicity and gestational age, standard risk calculators will yield catastrophically high false-positive rates for Trisomy 21. This tool applies the FMF median correction factors stratified by chorionicity, and optionally by gestational age window.

When to use

Adjusting raw laboratory MoM values for a twin pregnancy before entering them into an aneuploidy risk calculator.
Verifying the corrected MoM values provided by a laboratory when the correction method is uncertain.

Formula

Adjusted MoM = Raw MoM / Correction Factor DC: PAPP-A factor ~2.10, hCG factor ~2.00 MC: PAPP-A factor ~1.70, hCG factor ~1.90

Pearls & pitfalls

Dichorionic (DC) twins have roughly double the placental mass, hence correction factors are ~2.0-2.1.
Monochorionic (MC) twins share a single placenta and have slightly lower MoM inflation (factors ~1.7-1.9).
PAPP-A correction factors vary slightly with gestational age (higher correction at earlier gestations).
In dichorionic twins discordant for aneuploidy, the affected twin's contribution to the biochemistry is diluted by the normal co-twin.
For DC twins, per-fetus risk assessment using individual NT and adjusted biochemistry is the standard approach.
For MC twins, both fetuses share the same karyotype, so a single combined assessment applies.
IVF/ICSI pregnancies may have slightly different biochemical profiles; some labs apply an additional correction.

Next steps

Use the Adjusted MoM values in your standard Down Syndrome risk assessment software. Remember that in twin pregnancies, the NT measurement must be assessed for EACH fetus individually.

Evidence

Spencer K, Nicolaides KH. Screening for trisomy 21 in twins using first trimester ultrasound and maternal serum biochemistry. BJOG. 2003;110:276-280. Prats P, et al. First trimester screening for Down syndrome in twin pregnancies. Prenat Diagn. 2014.

References

Spencer K, Nicolaides KH. Screening for trisomy 21 in twins. BJOG. 2003;110:276-280.
Fetal Medicine Foundation. First trimester screening in multiple pregnancies. 2014.

First Trimester Twin MoM Adjustment

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