C. 1 in 30 – Explanation
| Down’s syndrome risk – 1/1,000 at 30 years then divide by 3 for every 5 years |
Down’s syndrome: epidemiology and genetics
Risk of Down’s syndrome with increasing maternal age
- 35 years = 1/350
- 40 years = 1/100
- 45 years = 1/30
One way of remembering this is by starting at 1/1,000 at 30 years and then dividing the denominator
by 3 (i.e. 3 times more common) for every extra 5 years of age
Cytogenetics
| Mode | % of cases | Risk of recurrence |
| Non-disjunction | 94% | 1 in 100 if mother < 35 years |
| Robertsonian translocation (usually onto 14) | 5% | 10-15% if mother is translocation carrier; 2.5% if father is translocation carrier |
| Mosaicism | 1% |
The chance of a further child with Down’s syndrome is approximately 1 in 100 if the mother is less
than 35 years
old. If the trisomy 21 is a result of a translocation the risk is much higher
