The synchrony of parturition at the population level has been studied for decades using various methods. To evaluate the tendency of individual mothers to synchronize their parturition dates with those of other mothers in the group, we propose a new parturition synchrony index (PSI). The PSI quantifies the degree of parturition synchrony of individual mothers within a group or population, regardless of the actual parturition date of the mother during the parturition season. A value of 1 indicates complete synchrony (i.e., the mother gave birth on the same day as the other mothers in the group), while values closer to 0 indicate an increasing degree of asynchrony in an individual mother’s parturition timing within the group. We evaluated the robustness of the PSI using simulated parturition data and further tested the influence of three key socio-ecological parameters on the PSI and its inter-individual variability: (1) the statistical distribution of parturition dates; (2) the duration of the parturition season; and (3) group size. We calculated the PSI for 140,490 individual mothers from 168 simulated maternal groups. For each group, we then calculated the average PSI and the coefficient of variation (CV) of the PSI values. Both metrics were influenced by all three tested socio-ecological parameters. Across all distribution types, the mean PSI was higher in shorter than in longer seasons. Only under lognormal and normal distributions was the mean PSI significantly higher in larger than in smaller groups. In contrast, the CV of the PSI was higher in longer seasons and in smaller groups across all distribution types examined. Our analyses demonstrate the broad applicability of the PSI across socio-ecological contexts. However, the reliability of the PSI is lower in extreme conditions, such as very short seasons or very small groups, where we observed wider confidence intervals. The PSI thus provides a robust and flexible tool for quantifying and functionally analyzing individual parturition synchrony in mammals and potentially in other taxa of viviparous vertebrates.