Common Birth Anomalies
Trisomy 21
Trisomy 21, also called Down syndrome, is a genetic condition that occurs at conception when the fetus receives three copies of chromosome 21 instead of two. Trisomy 21 gives rise to intellectual and physical disabilities for which it is impossible to predict the degree of severity. Trisomy 21 occurs in about 1 in 800 births1, however the incidence rate varies significantly with age. As a woman gets older, her risk of having a pregnancy with a chromosome abnormality increases. However, any woman, regardless of her age, has a risk of having an affected pregnancy. According to the Canadian Down Syndrome Society, Down Syndrome occurs in about 1 in 700 births, however the incidence rate varies significantly by age.
| Maternal Age and Risk of Trisomy 21 | |
|---|---|
| Maternal Age | Risk of Trisomy 21 at Birth |
| 20 | 1 in 1,530 |
| 25 | 1 in 1,350 |
| 30 | 1 in 900 |
| 32 | 1 in 660 |
| 34 | 1 in 450 |
| 35 | 1 in 360 |
| 36 | 1 in 280 |
| 38 | 1 in 170 |
| 40 | 1 in 100 |
| 42 | 1 in 55 |
| 44 | 1 in 30 |
According to the Canadian Down Syndrome Society,
80% of babies
born with a trisomy are from mothers aged 35 years and under.
Trisomy 18
Trisomy 18, also called Edwards syndrome, is a genetic condition that occurs at conception when the fetus receives three copies of chromosome 18 instead of two. In most cases, the fetus does not survive to birth. It is estimated that trisomy 18 occurs in approximately 1 out of every 6,000 births2. The incidence rate of trisomy 18 also increases with maternal age.
1Canadian Down Syndrome Society
2Ontario Ministry of Health and Long-Term Care
