Maternal age
Risk for trisomy 21
- Increases with maternal age.
- Decreases with gestational age because about 30% of affected fetuses die between the 12th and 40th week of pregnancy.
For example, in a woman who is 35 years old the risk for trisomy 21 at the 12th week of pregnancy is 1 in 250, but the chance that she will deliver an affected baby at 40 weeks is 1 in 350.