Chance of Chromosome Differences Based on Age
Trisomy 21 and trisomy 18 can happen at any age. The chance increases as the pregnant individual gets older, as shown in the table below. For example, a 29 year old has a 1 in 1095 chance (approximately 0.1%) to have a baby with trisomy 21. At age 40, the chance increases to 1 in 98 (approximately 1%)
| Age of Pregnant Individual / Age of Egg Donor * | Chance of Trisomy 21 (at birth) | Chance of Trisomy 18 (at birth) |
|---|---|---|
| 21 | 1/1470 | 1/15291 |
| 22 | 1/1452 | 1/15038 |
| 23 | 1/1430 | 1/14684 |
| 24 | 1/1400 | 1/14245 |
| 25 | 1/1362 | 1/13699 |
| 26 | 1/1315 | 1/13055 |
| 27 | 1/1255 | 1/12270 |
| 28 | 1/1182 | 1/11351 |
| 29 | 1/1095 | 1/10341 |
| 30 | 1/994 |
1/9234 |
| 31 | 1/881 | 1/8084 |
| 32 | 1/759 | 1/6935 |
| 33 | 1/634 | 1/5831 |
| 34 | 1/513 | 1/4812 |
| 35 | 1/403 | 1/3905 |
| 36 | 1/308 | 1/3121 |
| 37 | 1/231 | 1/2463 |
| 38 | 1/172 | 1/1923 |
| 39 | 1/129 | 1/1489 |
| 40 | 1/98 | 1/1145 |
| 41 | 1/76 | 1/875 |
| 42 | 1/61 | 1/667 |
| 43 | 1/50 | 1/506 |
| 44 | 1/43 | 1/383 |
| 45 | 1/38 | 1/289 |
*Ages at expected time of birth.
| Notes |
| Adapted from: JK Morris and NJ Wald. J Med Screen 2007;14:5-7 |
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