[Risk for Down syndrome based on maternal ages grouped in intervals of 2 and 5 years in the Mexican population].

Based on the analysis of 515 patients with Down's syndrome, identified in the Mexican program of Registry and Epidemiological Surveillance of External Congenital Malformations, we obtained the risk at birth for this disease, according to the mother's age, grouped in two and five years intervals. The results confirms other studies suggesting that the risk increases after age 30 and particularly 35. For example the incidence in women 21-22 years old is 0.62 per 1000 births and 40.7 in mothers ages 43-44. The data presented can be utilized for genetic counselling in Mexico and we recommend the use of the biannual figures because they are more precise than the usually employed five years intervals results.