There is no clear answer as to why Down Syndrome occurs and there is no way to prevent it. Several factors have been identified that may contribute. One is increasing maternal age due to a woman's aging eggs. Another may be the father's sperm and increasing age. This chart lists the approximate frequency of babies with Down Syndrome based on the mother's age at delivery. It is interesting to note that 75% of Downs babies are born to mothers under 35.
| Maternal Age in Years | *Estimated Risk |
Maternal Age in Years |
*Estimated Risk |
| 20 | 1/1231 | 35 | 1/274 |
| 21 | 1/1145 | 36 | 1/213 |
| 22 | 1/1065 | 37 | 1/166 |
| 23 | 1/1000 | 38 | 1/129 |
| 24 | 1/942 | 39 | 1/100 |
| 25 | 1/887 | 40 | 1/78 |
| 26 | 1/842 | 41 | 1/61 |
| 27 | 1/798 | 42 | 1/47 |
| 28 | 1/755 | 43 | 1/37 |
| 29 | 1/721 | 44 | 1/29 |
| 30 | 1/685 | 45 | 1/22 |
| 31 | 1/650 | 46 | 1/17 |
| 32 | 1/563 | 47 | 1/13 |
| 33 | 1/452 | 48 | 1/10 |
| 34 | 1/352 | 49 | 1/8 |
* These figures vary from source to source.