prenatal_diagnosis Flashcards
What is a prenatal diagnosis?
Prenatal diagnosis involves tests that help detect congenital malformations and genetic disorders before birth.
What are the indications for a prenatal diagnosis?
Indications include advanced maternal age, previous babies with chromosomal anomalies, one of the parents having balanced chromosomal translocations, family history of inherited conditions, exposure to teratogens, and more.
What are the characteristics of a good screening procedure?
A good screening procedure should be simple, inexpensive, and completely safe.
What are examples of screening tests?
Examples include family history, ultrasound scan, and maternal serum biochemistry testing.
What are examples of specific examinations (tests) that may carry risks?
Examples include amniocentesis, chorionic villus sampling (CVS), and fetal blood sampling.
When does screening for prenatal conditions begin?
Screening should begin at the first antenatal visit or during preconception counseling.
What factors are considered in the history-taking for prenatal screening?
Factors include maternal age, medical history, drug history, alcohol intake, family history, and previous pregnancy outcomes.
What genetic diseases are common in different ethnic groups?
Sickle cell disease is common in Africans, alpha thalassemia in Southeast Asians, beta thalassemia in Mediterranean Europeans and Indians, cystic fibrosis in Caucasians, and Tay-Sachs disease in Ashkenazi Jews.
What is the purpose of a first-trimester ultrasound scan?
It assesses fetal nuchal translucency thickness (>2.5mm is linked to chromosomal anomalies like Down syndrome).
When is the second-trimester ultrasound scan performed, and what does it detect?
It is done at 18-20 weeks to detect major malformations and at 22 weeks for cardiac malformations.
What is the purpose of a third-trimester ultrasound scan?
It helps detect urinary tract abnormalities, plan fetal surgery, and optimize postnatal management.
What is the mainstay of non-invasive screening for chromosomal anomalies and neural tube defects?
Maternal blood tests, particularly maternal serum alpha-fetoprotein (MSAFP).
When is maternal serum alpha-fetoprotein (MSAFP) measured, and what is the cut-off value?
MSAFP is measured between 15-20 weeks; a value above 2-2.5 multiples of the median (MOM) is considered elevated.
What should be done if MSAFP is elevated?
If elevated, a diagnostic ultrasound is indicated.
What are the causes of high MSAFP?
Causes include miscalculated gestational age, multiple pregnancy, threatened abortion, intrauterine fetal death, neural tube defects, anterior abdominal wall defects, congenital nephrosis, and teratomas.
What tests are used to screen for Down syndrome (Trisomy 21)?
The triple test (MSAFP, unconjugated estriol, beta HCG) and the quadruple test (adds inhibin A).
What are the expected results of the triple test in Down syndrome?
MSAFP and unconjugated estriol are low, beta HCG is high.
What is the detection rate and false positive rate of the triple test for Down syndrome?
Detection rate: 75%, false positive rate: 8.5%.
What is the detection rate and false positive rate of the quadruple test for Down syndrome?
Detection rate: 79%, false positive rate: 7.5%.
What are the expected results of screening tests for Edward syndrome (Trisomy 18)?
MSAFP, unconjugated estriol, and beta HCG are all low.
When are PAPPA and beta HCG typically measured?
PAPPA and beta HCG are measured between 11 weeks to 13 weeks, 6 days; the quadruple test may be done up to 20 weeks for late bookers.
What are the main invasive prenatal diagnostic procedures?
Amniocentesis, chorionic villus sampling (CVS), and cordocentesis (PUBS).
How is amniocentesis performed, and when is it typically done?
It involves inserting a 20/22 gauge spinal needle under ultrasound guidance, typically performed at 16 weeks (15-20 weeks), with results in 2-3 weeks.
What is the miscarriage risk associated with amniocentesis?
0.5-1%.
What are the advantages of chorionic villus sampling (CVS)?
It provides an earlier and faster karyotype report.
How is CVS performed, and when is it typically done?
It involves sampling trophoblast cells via a cannula through the cervix or a needle through the abdomen, typically done at 10-12 weeks.
What is the miscarriage risk associated with CVS?
2-6%.
What is cordocentesis (PUBS), and when is it typically performed?
It is done at 19 weeks and usually targets the umbilical vein at the cord insertion into the placenta.
What is the miscarriage rate associated with cordocentesis?
0.01
What are the indications for cordocentesis?
Indications include prenatal diagnosis of congenital disorders, hemoglobinopathies, rapid karyotype analysis, coagulopathies, and inborn errors of metabolism.
What are the benefits of identifying fetal anomalies in utero?
Allows choice of pregnancy termination, preparation for disability, specialized birth management, and intrauterine treatment.
