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| Age (years) | Frequency of Fetuses with Down Syndrome to Normal Fetuses at 16 weeks of pregnancy |
Frequency of Live Births of Babies with Down Syndrome to Normal Births |
|---|---|---|
| 15 - 19 | ---- | 1 / 1250 |
| 20 - 24 | ---- | 1 / 1400 |
| 25 - 29 | ---- | 1 / 1100 |
| 30 - 31 | ---- | 1 / 900 |
| 32 | ---- | 1 / 750 |
| 33 | 1 / 420 | 1 / 625 |
| 34 | 1 / 325 | 1 / 500 |
| 35 | 1 / 250 | 1 / 350 |
| 36 | 1 / 200 | 1 / 275 |
| 37 | 1 / 150 | 1 / 225 |
| 38 | 1 / 120 | 1 / 175 |
| 39 | 1 / 100 | 1 / 140 |
| 40 | 1 / 75 | 1 / 100 |
| 41 | 1 / 60 | 1 / 85 |
| 42 | 1 / 45 | 1 / 65 |
| 43 | 1 / 35 | 1 / 50 |
| 44 | 1 / 30 | 1 / 40 |
| 45 and older | 1 / 20 | 1 / 25 |
| The numbers are approximated and rounded. Using this data, geneticists have set the number separating low-risk from high-risk at 1 / 250. Why is there a difference in frequencies between 16 weeks and time of birth? Because of the spontaneous miscarriages of pregnancies with Down syndrome between these times. For information on risks of more detailed situations (such as translocation or mothers who have had previous babies with Down syndrome, see Dr. Paul Benke's essay on Risk and Recurrence of Down syndrome. Reference for the above table: Hook EB. JAMA 249:2034-2038, 1983. |