TABLE 1. Incidence of Down syndrome in live-borns and fetuses in relation to maternal age.
| Maternal age (years) | Incidence | ||
| At birth | At amniocentesis (16 weeks) | At chorionic villus sampling (9-11 weeks) | |
| 15-19 | 1/1250 | - | - |
| 20-24 | 1/1400 | - | - |
| 25-29 | 1/1100 | - | - |
| 30 | 1/900 | - | - |
| 31 | 1/900 | - | - |
| 32 | 1/750 | - | - |
| 33 | 1/625 | 1/420 | - |
| 34 | 1/500 | 1/325 | - |
| 35 | 1/350 | 1/250 | 1/240 |
| 36 | 1/275 | 1/200 | 1/175 |
| 37 | 1/225 | 1/150 | 1/130 |
| 38 | 1/175 | 1/120 | 1/100 |
| 39 | 1/140 | 1/100 | 1/75 |
| 40 | 1/100 | 1/75 | 1/60 |
| 41 | 1/85 | 1/60 | 1/40 |
| 42 | 1/65 | 1/45 | 1/30 |
| 43 | 1/50 | 1/35 | 1/25 |
| 44 | 1/40 | 1/30 | 1/20 |
| 45 and over | 1/25 | 1/20 | 1/10 |
Source: Thompson et al. (3).