Chap 17: prenatal diagnosis and screening Flashcards
- The fetus possesses a blood cell alloantigen derived from the father
- Not present on the maternal blood cells
- What is this called
- Fetomaternal alloimmunization (rhesus D and thrombocytopenia
- Blood antibodies of the igG class will cross the placenta
- Bind to their congnate alloantigen
- Blood cell descruction (lysis)
- Results in anaemia and thrombocytopenia
A couple has a child with Down syndrome, who has a 21q21q translocation inherited from the mother. Could prenatal diagnosis be helpful in the couple’s next pregnancy? Explain
- No, prenatal diagnosis would not be helpful for the couple’s next pregnancy
- Outcomes would include trisomy 21 or monosomy 21 (which is always lethal)
- Should consider counceling for alternative for having children
Cultured cells from a chorionic villus sample show two cell lines, 46,XX and 46,XY. Does this necessarily mean the fetus is abnormal? Explain.
- No
- This can indicate maternal cell contamination
What two chief types of information about a fetus can be indicated (although not proved) by assay of alpha-fetoprotein, human chorionic gonadotropin, and unconjugated estriol in maternal serum during the second trimester?
- The level of maternal serum alpha-fetoprotein (MSAFP) is typically elevated when the fetus has an open neural tube defect.
- The levels of MSAFP and unconjugated estriol are usually reduced and the human chorionic gonadotropin is usually elevated when the fetus has Down syndrome.
A couple has had a first-trimester spontaneous abortion in their first pregnancy and requests counseling.
Assuming that there are no other indications, should this couple be offered prenatal diagnosis for their next pregnancy?
- Prenatal diagnosis or karyotyping of the parents is usually not indicated after a single miscarriage
- most practitioners would offer parental chromosome analysis and prenatal diagnosis after three spontaneous unexplained miscarriages (although some practitioners suggest offering testing after only two) provided there are no other indications.
A couple has had a first-trimester spontaneous abortion in their first pregnancy and requests counseling.
What proportion of all pregnancies abort in the first trimester?
- Approx 15%
- A young woman consults a geneticist during her first pregnancy. Her brother was previously diagnosed with Duchenne muscular dystrophy and had since died. He was the only affected person in her family. The woman had been tested biochemically and found to have elevated creatine kinase levels, indicating she is a carrier of the disease.
Unfortunately, no DNA analysis had been conducted on the woman’s brother to determine what type of mutation in the DMD gene he had. The woman was investigated by molecular analysis and found to be heterozygous (A1/A2) for a microsatellite marker closely linked to the DMDgene. No relatives except the parents of the woman were available for analysis.
Can the phase of the mutation in the woman be determined from analysis of the available individuals?
- Yes
- Because her brother was affected, he could have only received the affected X from his mother.
- The phase of the mutation can be determined by analyzing the father’s X chromosome which is the normal X
- Fetal glycoprotein produced mainly in the fetal liver
- Secreted into the fetal circulation
- Excreted through the kidneys into the amniotic fluid
- amniotic fluid Alpha feto protien (AFAFP)
- Can be used to assess open neural tube defects
- What are some causes of elevated AFAFP other than open neural tube defects? (6)
- Normal variation
- overestimation of gestational age
- Ventral wall defects :gastroschesis and omphalocele
- contamination with fetal blood
- fetal death
- twin pregnancy
Without NIPS combined first and second trimester screening can detect what pecentage of Trisomy 21.
How does this improve with NIPS?
- Approx 95% with a 5% false positive rate
- With NIPS sensitivity improves to about 99% and <1% false positive rate
Suppose the frequency of Down syndrome is 1/600 in pregnancies in women younger than 35 years.
Assuming that a population of 600,000 women younger than 35 years are pregnant:
- How many pregnancies will be affected?
- 1000 affected pregnancies
- 600,000 / 600
Suppose the frequency of Down syndrome is 1/600 in pregnancies in women younger than 35 years.
Assuming that a population of 600,000 women younger than 35 years are pregnant:
All pregnant women undergo a sequential screening strategy, as follows: All participate in first-trimester screening with PAPP-A, hCG, and nuchal translucency. Sensitivity is 84% with a false-positive rate of 5%. Those who score positive are offered CVS,
- How many affected pregnancies will have CVS done?
- 1000 affected pregnancies
- 84% of 1000
- Answer: 840
