Clinical implications of atypical chromosome abnormalities diagnosed prenatally

Objective: To determine the frequency of atypical aneuploidy resulting from prenatal testing and assess the implications of these diagnoses on prenata l decision making. Methods: We reviewed all amniotic fluid and chorionic villus samples obtain ed between January 1994 and September 1997 and grouped the abnormal cases i nto typical or atypical subcategories. This distinction was; based upon whe ther the diagnosis provided a straightforward range of prognoses or an ambi guous clinical implication. Results were stratified by sample source to det ermine whether atypical aneuploidy was more commonly seen in cultures of ch orionic villi or amniocytes. We also evaluated the influence of ultrasound findings on prenatal decision making in atypical aneuploid cases. Results: Of 2960 samples, 134 were abnormal (4.4%), with 27 of 134 abnormal ities (20%) representing atypical aneuploidies. The percentages of chorioni c villus and amniocentesis cases complicated by atypical aneuploidy (22% an d 78%, respectively) were consistent with the distribution of procedures in the entire study. Ultrasound abnormalities did not invariably prompt a dec ision to terminate pregnancy (only two terminations of six fetuses with con genital malformation), whereas atypical karyotypes led to termination even in the presence of normal-appearing fetal anatomy (five terminations of 21 without malformations; P = .63). Conclusion: The frequency of atypical aneuploidy resulting from prenatal di agnosis was approximately 1.0%, and these cases represented 20% of all abno rmal karyotypes observed. The ambiguity conferred by atypical aneuploidy ca n influence a family's decision making, even in the presence of normal ultr asound findings. (Obstet Gynecol 1999;94:925-8. (C) 1999 by The American Co llege of Obstetricians and Gynecologists.).