222 Obstetrics and Gynecology Board Review •••
❍ What are the usual indications for prenatal diagnosis (eg, CVS, amniocentesis)?
Advanced maternal age, abnormal maternal serum screen, ultrasound finding, parental or family history of genetic
condition, or chromosome abnormality.
❍ What are the common chromosome abnormalities in humans?
There are aneuploidies, polyploidies, and structural alterations. Aneuploidy refers to numeric abnormalities. These
can be due to (i) nondisjunction: one pair of chromosomes fails to separate at anaphase resulting in one daughter cell
having both parts of the pair and the other having none; (ii) anaphase lag: one chromosome of a pair moves slower
during anaphase so its material is lost; (iii) polyploidy: the total number of chromosomes is duplicated more than
once, for example, 69 chromosomes. Structural alterations include (i) deletion: losing a portion of a chromosome;
(ii) duplication: there is an extra portion of a chromosome; (iii) insertion: a portion of a chromosome is attached to
another; (iv) inversion: the order of placement of a genetic material is inverted in the chromosome; (v) translocation:
portions of genetic material are removed from one chromosome and inserted onto another.