220 Obstetrics and Gynecology Board Review •••
❍ What factors affect the interpretation of a first trimester screen?
Maternal age, race, weight, multiple gestation, maternal IDDM, previous pregnancy with Down syndrome.
❍ If arthrogryposis is seen on ultrasound, what genetic syndromes should be considered?
Spinal muscular atrophy (SMA) and myotonic dystrophy.
❍ What is the risk in the general population for your patient to be a carrier of SMA?
Approximately 1/35.
❍ Your patient’s SMA results come back showing that she has 2 copy number of the SMN1 gene. What does
this mean?
This means she has a decreased risk to be a carrier. The more copy numbers a patient has, the less likely she is to be
a carrier. (If she had one copy of SMN1, then she would be a carrier.)
❍ Your patient’s CVS results show 47, XY +21[3]/46, XY[17]. What does this mean for the patient’s pregnancy?
And what other tests, if any, should be offered?
Possibilities with this result include true fetal mosaicism for Down syndrome, confined placental mosaicism (CPM)/
normal fetus, and fetus with full trisomy 21. Amniocentesis should be offered to distinguish between these possibilities.
❍ How often does mosaicism occur in amniocentesis or CVS samples?
0.25% amniocentesis and 1% CVS.
❍ An amniocentesis result reveals 46 total chromosomes with an inversion of chromosome 22. What follow-up
testing on the parents should be recommended?
Parental karyotype to determine if the results are de novo or inherited. If one of the parents has the same inversion
and is clinically normal, there is a high probability that the fetus will be unaffected (like the parent).
❍ What is the risk of pregnancy loss from amniocentesis?
1 in 300 to 500 if performed after 15 weeks’ gestation.