Clinical implications of atypical chromosome abnormalities diagnosed prenatally

To determine the frequency of atypical aneuploidy resulting from prenatal testing and assess the implications of these diagnoses on prenatal decision making.

We reviewed all amniotic fluid and chorionic villus samples obtained between January 1994 and September 1997 and grouped the abnormal cases into typical or atypical subcategories. This distinction was based upon whether the diagnosis provided a straightforward range of prognoses or an ambiguous clinical implication. Results were stratified by sample source to determine whether atypical aneuploidy was more commonly seen in cultures of chorionic villi or amniocytes. We also evaluated the influence of ultrasound findings on prenatal decision making in atypical aneuploid cases.

Of 2960 samples, 134 were abnormal (4.4%), with 27 of 134 abnormalities (20%) representing atypical aneuploidies. The percentages of chorionic villus and amniocentesis cases complicated by atypical aneuploidy (22% and 78%, respectively) were consistent with the distribution of procedures in the entire study. Ultrasound abnormalities did not invariably prompt a decision to terminate pregnancy (only two terminations of six fetuses with congenital malformation), whereas atypical karyotypes led to termination even in the presence of normal-appearing fetal anatomy (five terminations of 21 without malformations; P = .63).

The frequency of atypical aneuploidy resulting from prenatal diagnosis was approximately 1.0%, and these cases represented 20% of all abnormal karyotypes observed. The ambiguity conferred by atypical aneuploidy can influence a family's decision making, even in the presence of normal ultrasound findings.