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

Related ObG Topics:

Down Syndrome / Trisomy 21: Clinical Findings and Prenatal Considerations

cfDNA vs. Routine Screening – How Do They Compare?

Nuchal Translucency – First Trimester Measurement