The risk of common aneuploidies by maternal age (1) is < 35 years: Trisomy 21 (1/591), trisomy 18 (1/2862), and trisomy 13 (1/4651) ≥ 35 years: Trisomy 21 (1/100), trisomy 18 (1/454), and trisomy 13 (1/1438) Most chromosomal disorders due to older maternal age involve an extra chromosome (trisomy), particularly trisomy 21 (Down syndrome).