Table 1. Number of cases eligible for statistical analysis in each age group.

Observed numbers of cases with common autosomal trisomies (trisomies 21, 18 and 13) and predicted maternal age-specific rates in each age group (at the time of amniocentesis).

Age at the time of amnio-centesis	Total cases	Observed abnormal cases				Predicted rates (per 1,000 fetuses)
		T21	T18	T13	Common autosomal trisomies	T21	Common autosomal trisomies
34	621	2	0	0	2	2.67	4.54
35	2,781	8	6	0	14	2.93	4.61
36	3,206	12	3	1	16	3.90	5.71
37	2,698	17	4	4	25	5.58	7.84
38	2,487	23	6	1	30	7.98	11.01
39	1,900	15	8	2	25	11.09	15.22
40	1,533	19	8	2	29	14.91	20.46
41	1,022	19	4	0	23	19.44	26.73
42	723	25	5	1	31	24.69	34.04
43	442	14	4	3	21	30.65	42.39
44	214	8	1	0	9	37.32	51.77
45	113	5	2	1	8	44.70	62.19
46	50	1	0	1	2	52.80	73.64
47	15	1	0	0	1	61.61	86.14
48	14	1	1	0	2	71.14	99.66
Total	17,819	170	52	16	238

T21: trisomy 21, T18: trisomy 18, T13: trisomy 13.