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.
Adapted from: JK Morris and NJ Wald. J Med Screen 2007;14:5-7
Contact Us
Prenatal Screening Ontario
CHEO Research Institute
Centre for Practice-Changing Research Building
401 Smyth Road
Ottawa, ON K1H 8L1
Information line
Phone: 613-737-2281
Toll-Free: 1-833-351-6490
Monday to Friday, 9:00 AM to 3:00 PM EST
Subscribe to PSO Updates
Subscribe to stay informed about the latest PSO news and prenatal screening updates in Ontario.
Virtual educational sessions on prenatal screening and nuchal translucency quality assurance are available upon request for health‑care providers.