Table 2.  Indication for invasive test that resulted in antenatal diagnosis of Down's syndrome before 24 weeks' gestation by district for 1994-9 District Detected antenatally Detected at/after birth Proportion detected antenatally % (95% CI) Total cases Serum screen Nuchal thickening Anomaly Scan Maternal age/ previous affected pregnancy Total A 16 0 11 6 33 25 57 (44 to 70) 58 B 6 5 4 4 19 14 58 (41 to 75) 33 A&B serum screen combined 22 5 15 10 52 39 57 (47 to 67) 91 C 1 20 8 8 37 33 53 (41 to 65) 70 D* 0 2 1 2 5 7 42 (14 to 70) 12 E 6 1 6 6 19 16 54 (38 to 71) 35 C,D,&E 35 years, nucal thickening, serum screen 7 23 15 16 61 56 52 (42 to 62) 117 F 1 7 10 20 38 23 62 (50 to 74) 61 G 2 1 2 10 15 23 40 (24 to 56) 38 F&G 35 years scan 3 8 12 30 53 46 54 (44 to 64) 99 H 0 0 0 5 5 11 31 (8 to 54) 16 Total 32 (10%) 36 (11%) 42 (13%) 61 (19%) 171 (53%) 152 (47%) 53 (48 to 58) 323 * Data for 1997-9.