Updated: 2/29/2020

Genetic Testing

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  • Snapshot
    • A 41-year-old G3P1 presents to clinic 11 weeks pregnant. She has a history of 1 miscarriage and 1 uncomplicated live birth with no complications 10 years ago. Her daughter is healthy. She has no medical conditions. Vital signs are within normal limits. She receives first-trimester screening and a cell-free DNA test.
  • Introduction
    • Overview
      • asseses if fetus has a genetic condition
  • Epidemiology
    • Incidence
      • 1/150 live births with chromosomal abnormalities
    • Risk factors
      • advanced maternal age
      • history of prior fetal aneuploidy or abnormality
  • Pathogenesis
    • Most from nondisjunction
    • Some due to somatic mosaicism or translocations
  • Studies
    • First-trimester screening
      • indications
        • between 10-14 weeks gestational age
      • serum labs
        • serum free β-hCG or total hCG
          • ↑ trisomy 21
          • ↑ trisomy 18
          • ↓ trisomy 13
        • pregnancy-associated plasma protein A (PAPP-A)
          • ↓ trisomy 21
          • ↓ trisomy 18
          • ↓ trisomy 13
      • imaging
        • ultrasound
          • nuchal translucency measurement
            • ↑ trisomy 21
            • ↑ trisomy 18
      • invasive studies
        • chorionic villous sampling (sampling of prenatal villi)
          • indications
            • abnormal first trimester screen
            • 10-13 weeks gestational age
              • confirm with ultrasound prior to testing
          • complications
            • transverse limb abnormality (< 10 weeks)
            • rupture of membranes
            • chorioamnionitis
            • miscarriage
            • Rh sensitization
    • Second-trimester screening
      • indications
        • 15-22 weeks gestation (best time 16-18 weeks)
      • serum labs
        • quadruple screen
          • hCG
            • ↑ trisomy 21
            • ↓ trisomy 18
          • alpha fetoprotein (AFP)
            • ↓ trisomy 21
            • ↓ trisomy 18
          • dimeric inhibin A
            • ↑ trisomy 21
            • no change or ↓ trisomy 18
          • unconjugated estriol
            • ↓ trisomy 21
            • ↓ trisomy 18
      • invasive studies
        • amniocentesis (sampling of amniotic fluid)
          • indications
            • abnormal quadruple screen
            • 16-20 weeks gestational age
              • confirm with ultrasound prior to testing
          • complications
            • rupture of membranes
            • chorioamnionitis
    • Sequential integrated screening
      • first trimester
        • imaging
          • ultrasound
            • nuchal translucency
        • serum labs
          • PAPP-A
      • second trimester
        • serum labs
          • quadruple screen
      • higher detection rates than either first or second trimester screening alone
    • Cell-free DNA
      • indications
        • to detect fetal Down syndrome
        • 10 weeks to term
        • pregnant patients at high risk for fetuses with Down syndrome
      • highest detection rate for Down syndrome
      • higher false-positive rate for low-risk patients

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