For the past three decades, there has been a consistent trend for delayed childbearing in the United States. The aims of this study were: 1) to compare the impact of advanced maternal age on the birth prevalence of Down syndrome in African Americans (AA), Mexicans (MX) and Non-Hispanic Whites (NHW); and 2) to determine whether racial/ethnic differences in the impact of advanced maternal age on the risk of Down syndrome might reflect differences in their use of amniocentesis. To answer our first question, we compared the odds ratios and the population attributable risks (PAR) of Down syndrome at birth in AA (N = 1,627,491), MX (N= 1,025,229) and NHW (N=6,724,305) using the National Center for Health Statistics birth data for the years 1989-1991. Both the odds ratio and the PAR of Down syndrome due to maternal age ≥ 35 years were highest for Mexicans [OR: 6.37, 95% C.I., 5.3 - 7.7); PAR = 27%], intermediate for AA [OR: 5.4, 95% C.I., 4.4-6.5); PAR = 20%] and lowest for NHW [OR: 3.2, 95% C.I., 3.0-3.5; PAR = 17%]. Advanced maternal age had a greater impact on birth prevalence of Down syndrome for MX and AA mothers even though a significantly lower proportion of MX and AA mothers were ≥ 35 years of age. Mantel-Haenszel analysis of age-specific rates of amniocentesis use for the three racial/ethnic groups showed that AA and particularly MX mothers were substantially less likely to use amniocentesis. In particular, AA and MX who were 35-39, 40-44 and 45-49 years of age had amniocentesis rates that were about 2/3 to 1/3 of the rates for their NHW counterparts [p-values< 0.0001]. Conclusions: 1) Advanced maternal age has a greater impact on the risk of Down syndrome for African American and particularly Mexican mothers. 2) The greater impact of maternal age on the risk of Down syndrome for Mexicans and African Americans may at least in part reflect the lower availability and/or use of amniocentesis for African Americans and particularly Mexicans in the United States.