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Grand Rounds

How Accurate are Current Algorithms for Aneuploidy Screening in Twin Pregnancies

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BACKGROUND AND PURPOSE:

  • Direct data on aneuploidy screening performance for twins in lacking
  • Algorithms are based on ‘proportional’ differences between singletons and twins
    • Assumptions based on differences between normal and affected singletons
  • Median first trimester levels for monochorionic affected twins (estimate)
    • PAPP-A: 0.80 MoM
    • Free β hCG levels: 3.76 MoM
  • Median first trimester levels for dichorionic affected twins (estimate)
    • PAPP-A: 1.33 MoM
    • Free β hCG levels: 2.93 MoM
  • Wald et al. (Obstetrics & Gynecology, 2018) sought to determine the performance of prenatal screening for Down syndrome in twin pregnancies based on empiric data

METHODS:

  • Data were collected from 5 centers
    • Twin pregnancies with Down syndrome and matched controls
    • NT | Chorionicity, PAPP-A | Free β hCG
  • Distribution parameters were determined and used to estimate screening performance

RESULTS:

  • 61 pregnancies were included
  • Monochorionic affected twin pregnancies
    • Free β hCG levels: 2.63 MoM (95% CI, 1.79‐22 MoM)
      • Lower than proportionality estimates
    • Dichorionic affected twin pregnancies
      • PAPP-A: 1.88 MoM (95% CI, 1.60‐17 MoM)
        • Higher than than proportionality estimates
      • Compared to detection rate of 86% with a 3% FPR (88% for 5% FPR) in singleton pregnancies for the combined test (NT, maternal age and serum markers), performance for twins was as follows
        • Monochorionic twin pregnancies: 87% detection (90% for 5% FPR)
        • Dichorionic twin pregnancies: 74% detection (79% for 5% FPR)

CONCLUSION:

  • The assumption that serum marker MoM can be simply derived from singleton pregnancies in incorrect
  • However, performance for monochorionic twins remains comparable to singleton pregnancies
  • For dichorionic twin pregnancies, combined test did not perform as well
  • Different risk cut-offs should be used based on chorionicity, which can be established with ultrasound early in pregnancy
  • In addition, the authors state

it is still appropriate to use PAPP-A and Free β hCG as well as nuchal translucency in screening twin pregnancies, but appropriate revisions are needed to update the distribution parameters for the screening markers

Learn More – Primary Sources:

Prenatal screening for Down syndrome in twin pregnancies: Estimates of screening performance based on 61 affected and 7302 unaffected twin pregnancies

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Related ObG Topics:

Down Syndrome / Trisomy 21: Clinical Findings and Prenatal Considerations
Practical info on evidence based medicine for your women's healthcare practice
cfDNA vs. Routine Screening – How Do They Compare?
Practical obstetrics info for your women's healthcare practice
Nuchal Translucency – First Trimester Measurement

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