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Handbook of Genetic Counseling/First Trimester Screening

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First Trimester Screening

Overview

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  • Screen low-risk population to identify those at higher risk to give birth to child with a chromosome abnormality
    • Normal result does not mean that there are no abnormalities
    • Abnormal result indicates that diagnostic testing by amniocentesis or CVS is warranted
  • Consists of ultrasound and blood test
  • Performed between 10th and 14th week of pregnancy

Procedure

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  • Biochemical screening
    • Blood draw done by finger stick onto filter paper
    • May be taken between 8 weeks, 4 days and 13 weeks, 3 days
    • Measures levels of substances found in the blood of all pregnant women
      • FreeBeta Human chorionic gonadotropin (hCG) measurements
        • Glycoprotein derived from the placenta
        • Serum levels tend to be high in patients carrying a fetus affected with Down Syndrome (average = 1.9 MoM)
        • Serum levels significantly reduced in patients carrying a fetus affected with Trisomy 18 (average = 0.18 MoM)
      • Pregnancy Associated Plasma Protein A (PAPP-A)
        • Produced by trophoblast of placenta
        • Significantly reduced in blood of women carrying fetuses with Down Syndrome (average = 0.44 MoM)
        • Significantly reduced in blood of women carrying fetuses with Trisomy 18 (average = 0.32 MoM)
  • Ultrasonagraphic findings
    • Measure nuchal translucency (NT)
      • Amount of fluid that accumulates behind the neck of the baby
        • Echo-free space between skin and soft tissue overlying cervical spine
        • Present only during first trimester
      • About 30-86% of fetuses with increased nuchal translucency have chromosomal abnormality
      • Increased in fetuses with chromosome abnormalities, cardiac defects, and certain genetic syndromes
        • Greater than 3.5 mm is considered out of normal range
        • Increased NT is not diagnostic of chromosome abnormality
        • Lymphedema seen in NT may be result of fetal cardiac abnormalities
      • Usually resolves in second trimester
      • No consensus on how to measure nuchal translucency resulting in conflicting results about effectiveness
    • Measured transbdominally in 95% of cases and transvaginally in other 5%
    • Procedure takes between 15 and 30 minutes

Detection Rate

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  • Biochemical screening without NT
    • Identifies greater than 60% of Down Syndrome
    • Identifies 90% of Trisomy 18
    • About 1 in 25 women with an increased risk based on screening will have baby with Down Syndrome
  • Biochemical screening with NT
    • Identifies about 90% of Down Syndrome
    • Identifies up to 97% of Trisomy 18
    • False positive rate is 5%
    • About 1 in 17 women with an increased risk based on screening will have baby with Down Syndrome
  • Cannot screen for open neural tube defects so AFP testing still recommended in 2nd trimester
  • Cannot identify all birth defects

Benefits and Limitations of Testing

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  • Benefits
    • Helps reduce maternal anxiety at earlier gestational age
    • Allows patient to have CVS for diagnosis in 1st trimester
    • Allows for 1st trimester termination of affected fetus
    • Not an invasive procedure - no risk to mother or to fetus
  • Limitations
    • Currently not covered by insurance
    • Only screens for two out of possible thousands of genetic conditions
    • Screening will miss about 10% of Down Syndrome
    • Still requires invasive procedure for diagnosis
    • Not as sensitive if multiple pregnancies
    • Patient specific risks rely on accuracy of clinical information that is interpreted for results of test

Research Study at Magee-Womens Hospital

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  • Eligibility criteria: women 18-45 between 10 and 14 weeks
  • To determine if first trimester screening is an effective method to detect Down Syndrome and other chromosome abnormalities
    • If risk higher than 1 in 270, can continue routine prenatal care, undergo CVS at 10-13 weeks, or have amniocentesis after 15 weeks
    • Women who choose to participate must provide information on pregnancy outcome
  • Responsible for all costs, including blood sample, ultrasound, and genetic counseling

References

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  • "ACOG Committee Opinion: First Trimester Screening for Fetal Anomalies with Nuchal Translucency." American College of Obstetrics and Gynecology. http://www.medem.com
  • "NIH News Release: First Trimester Screening for Down Syndrome Possible, NICHD-Funded Study Finds." National Institutes of Health. http;//www.nih.gov
  • "Screening for Down Syndrome, Trisomy 18 Syndrome, and Open Neural Tube Defects in the First Trimester." GeneCare Medical Genetics Center". http://www.genecare.com
  • Spencer K. "Accuracy of Down Syndrome Risks Produced in a First-Trimester Screening Programme Incorporating Fetal Nuchal Translucency Thickness and Maternal Serum Biochemistry." Prenatal Diagnosis (2002) 22;244-246.

Notes

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The information in this outline was last updated in June 2002.

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