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Handbook of Genetic Counseling/Trisomy 18

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Trisomy 18

Contracting

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  • What do you know about why you have been referred?
  • What do you know about the triple screen? What is your understanding of your test results?
  • What questions or concerns would you like to address today?
  • Overview of session:
    • Family and medical history
    • Explanation of maternal serum screening
    • Overview of results and what they mean
    • Explanation of genes and chromosomes
    • Information on Trisomy 18
    • Options for the future

Prenatal Intake

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  • Important to obtain information on pregnancy and family history to help assess potential risks to pregnancy
  • Prenatal intake
    • How many times have you been pregnant? Any previous miscarriages or stillbirths?
    • History of infertility?
    • LMP beginning of November? EDC middle of August?
    • Any illness - infections, colds, fever, rashes?
    • Any bleeding?
    • Any exposures - X-rays (including dental), smoking, alcohol, recreational drugs, medications?
    • Any problems with pregnancy?
    • Personal background - occupation, religion, ethnicity, chronic illnesses for consultand and partner
    • Consanguinity?
  • Family history
    • Review information obtained previously
    • Any history of miscarriages, infertility, stillbirths, birth defects?
    • Other chronic illnesses, learning problems, health problems that we did not discuss?

Maternal serum screening

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  • Background
    • Screening test
      • Not a diagnostic test
      • Used to identify women who are at increased risk for certain birth defects
        • These women then offered diagnostic testing
        • Negative or "normal" result does not mean that a child will not have a birth defect
        • Positive result is not a diagnosis of an abnormality
      • Can identify women at increased risk for Trisomy 21, Trisomy 18, or open neural tube defects
    • Blood test
      • Fetal and placental products can be detected in mother's blood serum during pregnancy
      • Can be offered from 14-22 weeks but most accurate at 16-18 weeks
  • Indirect measure of levels of three substances in mother's blood
    • Alpha-fetoprotein (AFP)
      • Produced by liver of fetus and excreted into amniotic fluid
      • Passes into mother's bloodstream and concentration rises gradually throughout pregnancy
    • Human chorionic gonadotropin (HCG)
      • Pregnancy hormone made by the placenta
      • Level peaks at 10 weeks and declines throughout pregnancy
    • Unconjugated estriol (uE3)
      • Hormone made by fetus and placenta
      • Level increases throughout pregnancy

Risk assessment

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  • Laboratory calculates a woman's risks based on levels of three substance plus other factors
    • Gestational age
      • Marker levels change throughout pregnancy
      • Inaccurate dates are common reason for false positive
      • Gestational age can be confirmed by ultrasound
    • Maternal age
    • Multiple gestations
    • Maternal weight
      • Increased weight means increased blood volume
      • Markers will be diluted in serum and their concentration lower
    • Maternal race
    • Diabetic status
      • Insulin-dependent diabetics have increased risk for neural tube defects
      • Tend to have lower AFP levels, so use lower AFP cutoff
  • Your specific results:
    • Every pregnancy has 3-5% risk for congenital malformation
      • Background risk
      • Most of these problems cannot be detected prenatally
    • As a woman's age increases, her risk for certain birth defects increases
    • Risk figures for your pregnancy:
      • ONTD: ________
      • Down Syndrome: ________ compared to age associated risk _______
      • Trisomy 18: ________ compared to age associated risk _______
    • Screen positive for increased risk for Trisomy 18
      • Occurs when AFP, hCG, and uE3 levels are all lower than expected
      • Evaluation of screening performance
        • Up to 100 of every 1,000 women who take test will have abnormal result (up to 10%)
          • Only 3 of those 100 women will have a baby with a birth defect
          • Most abnormal test results indicate dates are wrong or another factor above has not been accurately accounted for
          • Important to remember that test cannot identify all birth defects
        • Abnormal result could be normal variation
        • Sometimes no explanation for abnormal results - increased risk for 3rd trimester complications
        • Test can detect:
          • 60% of Trisomy 18
          • 80% of ONTD
          • 60% Down Syndrome

Genes and Chromosomes

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Trisomy 18

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  • Clinical picture
    • Can cause heart problems, kidney problems and other health issues
    • Usually fatal within first year of life
    • Children that survive have severe intellectual disability
  • Increased risk for pregnancy loss

Options for further testing

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  • Ultrasound
    • Confirm gestational age or identify multiple fetuses to help interpret triple screen results
    • Can detect many major birth defects - rule out 95% of NTDs if visualization is not limited
    • Many fetuses with Trisomy 18 have certain "markers" that can be identified on ultrasound
      • Growth retardation
      • Single umbilical artery
      • Heart problems
      • Polyhydramnios
    • Can't diagnose chromosomal abnormalities
  • Amniocentesis
    • Performed after 15 weeks
    • Diagnostic test for some conditions
      • 99.7% accuracy for fetal chromosome analysis
      • Detects 96% of open neural tube defects by testing AFAFP
      • Cannot detect all birth defects or mental retardation
    • Risk of pregnancy loss due to procedure is 0.5%
    • Explain amniocentesis if interested
      • Insert needle into abdomen through uterus to take sample of amniotic fluid that contains cells from fetus used for karyotyping
      • Procedure:
        • Ultrasound to locate fetus and placenta
        • Abdomen cleaned with betadeine and local anesthetic (xylocaine) used to numb outer layer of skin
        • Physician inserts needle in abdomen through uterus under ultrasound guidance
          • Needle inserted 1-2 minutes
          • May feel some discomfort similar to menstrual cramp
        • Fluid is sent to lab and results ready in 1-2 weeks
      • Recommendations
        • No strenuous activity for 24 hours
        • Notify doctor if severe cramping, bleeding or leakage of fluid because there is very small risk of infection
        • Rh negative mothers should take RhoGAM

Potential Psychosocial Issues

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  • Emphasize the importance of choice
  • Religious beliefs impact decisions about testing and termination
  • Anger, distrust because of family physicians advice
  • Anxiety, sadness, disbelief, fear concerning possibility of Trisomy 21
    • Provide reassurance when possible
    • Explain benefits of prenatal diagnosis to help prepare emotionally, mentally, and socially for birth of child with chromosome abnormality
  • False positive may cause unnecessary anxiety

Support Groups

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Trisomy 18 Foundation at www.trisomy18.org

References

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  • Creasy RK, and Resnik R, eds. (1994) Maternal-Fetal Medicine 62-84.
  • "Maternal Blood Screening." (2001) March of Dimes Fact Sheet. http://www.modimes.org.
  • Gardner RJM, and Sutherland GR. (1996) Chromosome Abnormalities and Genetic Counseling 325-371.
  • Milunsky A, ed. (1998) Genetic Disorders and the Fetus: Diagnosis, Prevention and Treatment 179-23**

Notes

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The information in this outline was last updated in 2002.

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