exam 1 - genetic testing Flashcards
A 41 yo G3 P1011 LMP 6 weeks ago presents for prenatal care and states, “I want to tell you I don’t believe in that genetic testing. This baby is a gift from God and I will accept them the way God made them; so those tests won’t have any impact on me. I don’t want them.”
You counsel the patient about available options for genetic testing, telling her that they has no obligation to have such testing, but you explain to her the possible benefits of being tested. After listening to what you have to say, she decides to undergo cell-free DNA testing.
complex issues to consider when recommending genetic testing
-Potential for “wrongful life” lawsuits in obstetric cases
-Rights of the disabled
-Sensitivity and specificity of genetic testing
-Potential for unnecessary invasive procedures based on results, especially with obstetrical testing
-Guilt about results
-document that pt declined and was educated
-Genetic discrimination
-Genetic Information Nondiscrimination Act of 2008 (GINA)
-Potential for employer discrimination
-Potential for discrimination with health, disability, and life insurance
-Privacy matters- HIPAA
statistics concerning chromosomal anomalies in pregnancy
-approx 1:150 live births have an abnormal phenotype
-about 20% of infant deaths are due to chromosomal abnormality
risk factors associated with genetic anomalies
-advanced maternal age (35+)
-advanced paternal age (40-50+)
-prior history of child with birth defect
-prior hx of child with genetic disorder
-parent with known genetic disorder
-parental carrier status of a genetic disorder
-pregnancy with identified structural anomaly identified by US
reasons by you genetic test
-if pt says they will love the baby no matter what
-you can halt or help the baby earlier or identify health problems earlier to help the child -> better outcomes
-down syndrome 1:700 -> heart anomaly are common -> help the child or be prepared
screening vs. dx testing
-understand the difference
-screening- identifies a group of pts at risk of a particular condition
-tolerable to the pt
-relatively inexpensive
-easy to perform
-high sensitivity and specificity
-dx testing renders a dx of a particular condition:
-invasive test is required
-preimplantation genetic dx
-chorionic villus sampling
-amniocentesis
preimplantation genetic testing
-Non-invasive testing:
-Uses cell-free DNA obtained during culture
-87-100% accurate
-Invasive testing:
-Performed only via embryos formed via IVF
-May be taken from cells from a very early gestation
-A polar body from the zygote
-A blastomere
-A cell from the blastocyst
-Primarily used to identify known genetic conditions in the family
-Down syndrome
-Turner syndrome
-muscular dystrophy and cystic fibrosis
-Chromosomal structural rearrangements
-also performed in AMA pts undergoing IVF for aneuploidy (abnormal chromosomes) -> Controversial bc many false positive results
-Confirmation with chorionic villus sampling or amniocentesis is recommended due to scarcity of tissue and potential for error
testing with chorionic villus sampling (CVS)
-1st trimester ONLY -> gives pt privacy if they decide to terminate pregnancy
-higher risk of loss at 2nd or 3rd -> DONT DO
-transcervical or transabdominal approach under US guidance
-results available in about a week
-1/3 of pts experience vaginal bleeding after transcervical CVS
-risk of infection or leakage of amniotic fluid is <0.5%
-loss of pregnancy (about 0.22% risk)
-used for abnormality on screening or US, or if pt prefers dx over screening
amniocentesis
-NEVER in 1st trimester bc risk of loss is much higher
-performed any time after 14 wks
-performed via transabdominal approach under US guidance
-fetal cells obtained in amniotic fluid
-karyotype is obtained
-results available in 7-14 days
-risk of pregnancy loss is approx 0.1-0.3%
-risk of leakage of amniotic fluid or vaginal bleeding is about 1-2%
-used for abnormality on screening or US, or if pt prefers dx over screening
karyotypes: of fetus with down syndrome (left) and of fetus with trisomy 13 (patau syndrome) (right)
obtained by CVS or amnio
when are results available for dx studies
-Preimplantation genetic dx: 1-2 days
-Karyotype: 7-14 days
fluorescence in situ hybridization
-Yields results in 24-48 hours for amniocentesis and CVS
-results are only available for abnormalities in chromosomes 13, 18, 21, X and Y
-targets certain genes
-identify sex of fetus
-Should not be considered dx
-should be confirmation of results before being certain of dx
aneuploidy
-having too many or to few chromosomes
-MC- down syndrome (trisomy 21) -> 1:700 live births
-other common trisomes -> Edwards syndrome (trisomy 18) and platau syndrome (trisomy 13) -> both are incompatible with life
-risk of trisomy 21 increases with maternal age -> but more babies are born to younger parturients bc there are more babies born to younger pts
chromosomal abnormalities in 2nd trimester pregnancies based on maternal age at term
-any chromosomal anomaly 1:122
-by 40 its 1:40
dysmorphic features of downs syndrome
-Short stature
-Single palmar crease
-Flat nasal bridge
-Protruding tongue
-Upslanting palpebral fissures
-Bilateral epicanthal folds
-Small ears
