Up to 19% of postpartum mothers experience depressive symptoms, which are associated with infant development. Thus, research examining postpartum depression has implications for mothers' and infants' well-being. However, this research relies on the often-untested assumption of measurement invariance-that measures capture the same construct across time and sociodemographic characteristics. In the absence of invariance, measurement bias may confound differences across time and group, contributing to invalid inferences. In a sociodemographically diverse (40.7% African American, 58.9% White; 67.9% below two times the federal poverty line; 19.4% with less than high school education), rural, longitudinal sample ( = 1,275) of mothers, we used moderated nonlinear factor analysis (MNLFA) to examine measurement invariance of the Brief Symptom Inventory-18 (BSI-18) Depressive Symptoms subscale across time since birth, racial group, education, income, primiparity, and maternal age at childbirth. We identified evidence of differential item functioning (DIF; i.e., measurement noninvariance) as a function of racial group and education. Subsequent analyses indicated, however, that the DIF-induced bias had minimal impacts on substantive comparisons examining change over time since birth and group differences. Thus, the presence of measurement noninvariance does not appear to bias substantive comparisons using the BSI-18 Depressive Symptoms subscale across the first 2 years since birth in a sample comprising primarily African American and White mothers living in predominately rural, low-income communities. This study demonstrates the importance of assessing measurement invariance and highlights MNLFA for evaluating the impact of noninvariance as a preliminary step that increases confidence in the validity of substantive inferences. (PsycInfo Database Record (c) 2023 APA, all rights reserved).