A multiple logistic regression model estimates the incidence of membranous VSD in the Nkx2-5+/- F2 population as a function of bearing a susceptibility genotype at none, one or more of the chromosomes 6, 8, and 10 loci and maternal age. The incremental risk associated with one month of maternal aging is small relative to a susceptibility genotype at any locus, but the cumulative risk associated with maternal aging can surpass any genetic effect, which remains constant. OR, odds ratio. CI, confidence interval.