••• Chapter 21^ Genetics for the Obstetrician^219
❍ Your patient is a fragile X premutation carrier. What is her risk to have an affected son?
50%.
❍ Your patient is a fragile X premutation carrier and is pregnant with a female fetus. What are the clinical
possibilities for this patient’s daughter?
She could be a premutation carrier (like her mother), the repeat size could expand to a full mutation (she could
have some clinical symptoms of fragile X), or she could be “normal” if she receives the typical X from her mother.
❍ What analytes are utilized in first trimester screening?
PAPP-A and beta hCG.
❍ At what gestational age, would you offer first trimester screening?
Between 10 and 14 weeks (each center may have a slightly different range).
❍ What conditions are screened for by the first trimester screen?
Trisomy 21 and trisomy 18 (some labs will report a risk of trisomy 13/trisomy 18 combined).
❍ If a patient has an increased nuchal translucency in the first trimester and a normal fetal karyotype on
chorionic villus sampling (CVS), what other tests would you offer?
Microarray and Noonan syndrome testing needs to be offered when there are normal chromosomes on CVS.
Anatomy scan/level 2 ultrasound, fetal echocardiogram (due to increased likelihood of congenital cardiac anomaly),
and MSAFP [as routine screen for open neural tube defects (ONTD)].
❍ If a cystic hygroma is seen on ultrasound, what is the likelihood of aneuploidy?
60% to 75% (most of these will be 45X).
❍ When does ACOG recommend fetal microarray be offered?
When there is abnormal anatomic findings and a normal conventional karyotype, or if there is a fetal demise with
congenital anomalies and a karyotype cannot be obtained.
❍ What conditions are screened for by second trimester blood screening?
Trisomy 21, trisomy 18, and ONTDs.
❍ What analytes are utilized for the quadruple screen?
AFP, unconjugated estriol, dimeric inhibin A, and hCG.
❍ What are the potential causes of an elevated AFP level in a second trimester blood screen?
Multiple gestations, IUFD, ventral wall defect, ONTD, incorrect dating of pregnancy, oligohydramnios, renal
agenesis, and congenital nephrosis.
❍ What factors affect the interpretation of the quadruple screen?
Maternal age, race, weight, maternal IDDM, gestational age, multiple gestations, and previous pregnancy with
ONTD.