# Handbook of Genetic Counseling/Trisomy 18

Trisomy 18

## Contracting

* 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

* 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

* 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

* 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

## Trisomy 18

* 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

* 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

* 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

Trisomy 18 Foundation at www.trisomy18.org

## References

* 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

The information in this outline was last updated in 2002.