Ultrasound findings before amniocentesis in selecting the method of analysing the sample.

OBJECTIVE To determine if the primary method of cytogenetic analysis in pregnant women undergoing amniocentesis should be quantitative fluorescent polymerase chain reaction (qf-PCR), with karyotyping being performed only on those with abnormal ultrasound findings. METHODS Amniocentesis was performed in 3854 cases. The median maternal age was 36 years and median gestational age was 18 weeks. The indication for karyotyping was an increased risk for aneuploidy in the absence or presence of sonographic abnormalities detected at the scan before amniocentesis. All samples were analysed by qf-PCR and full karyotyping. For each detectable fetal defect, the positive or negative likelihood ratio for aneuploidy was determined. OUTCOME MEASURE Detection rate of clinically significant chromosomal abnormalities. RESULTS The karyotype was normal in 3617 (93.9%) cases. In 237 (6.1%) cases, the karyotype was abnormal and the detection rate by qf-PCR was 92.4%. A policy of performing qf-PCR in all cases and karyotyping in only those with combined likelihood ratios of > 1, > 3, and > 5 would detect 98.3, 96.6, and 95.4% of all chromosomal abnormalities and would require karyotyping in 16.1, 8.0, and 5.4% of the cases, respectively. CONCLUSIONS More than 95% of the aneuploidies can be detected if karyotyping is performed in addition to qf-PCR in about 15% of the cases selected on the basis of ultrasound findings before amniocentesis.