AGE-SPECIFIC INCIDENCES OF CHROMOSOME-ABNORMALITIES AT THE 2ND-TRIMESTER AMNIOCENTESIS FOR JAPANESE MOTHERS AGED 35 AND OLDER - COLLABORATIVE STUDY OF 5484 CASES

The aim of this study was to calculate the expected incidences of chro mosome abnormalities found at amniocentesis in Japanese women aged 35 and older. From four clinics in Japan, we gathered genetic amniocentes is data on 5484 pregnant women at risk only due to their advanced age, 35 years and older. We analyzed the data using the logistic regressio n model. Of the 5484 fetuses, 117 (2.1%) were diagnosed with a chromos ome abnormality. The abnormal karyotypes included 42 cases of trisomy 21; 13 of trisomy 18; 7 of trisomy 13; 10 of 47,XXY; 4 of 47,XXX; 1 of 47,XYY; 27 with various structural aberrations; and 13 with various t ypes of mosaicism. The incidences of trisomy 21, lethal autosomal aneu ploidies (trisomy 18 and trisomy 13), and sex-chromosome abnormalities (XXY, XXX, XYY) increased with maternal age. Parameters of the regres sion equations with their standard errors were calculated and the expe cted incidences of chromosome abnormalities at each maternal age were derived. The expected incidences of chromosome abnormalities obtained in this study are the first data published for Japan and will be usefu l for the counseling of pregnant women. The incidence of trisomy 21 is not different from the rates published previously for Western countri es. The incidences of chromosome abnormalities are not affected by rac e or by geographic factors.