The “triangular” pattern of reproduction in arvicoline rodents predicts small fecundity early and late in life and high fecundity in the middle. In producing this pattern, the effects of maternal age and parity are typically highly confounded. We present results of a laboratory study designed to disentangle these effects in the common vole (Microtus arvalis) by analyzing variation in sizes of the first 2 litters born to 3 age classes of females subjected to photoperiods of 8 h and 16 h of light. Although the youngest females, paired at 2 weeks of age, decreased size of the 2nd litter in either light environment, the older 2 classes, paired at 3–4 months and 1 year of age, increased their 2nd litters, as predicted by the triangular pattern. This age-specific effect of parity could be explained by differences in the size of 1st litters.