Background Perinatal depression negatively impacts women, parenting, and children's development. However, not much is known about maternal specific beliefs that may be associated with perinatal depression. We created a new measure that examined the rigidity of perinatal women's beliefs in three major domains suggested to be closely related to mood and behavior: anticipated maternal self‐efficacy, perceptions of child vulnerability, and perceptions of societal expectations of mothers (PSEM). Methods A 26‐item measure (the Rigidity of Maternal Beliefs Scale, RMBS) was developed and completed by women at two time points, pregnancy (n = 134) and postpartum (n = 113), along with the Edinburgh Postnatal Depression Scale. Exploratory factor analysis (EFA) examined the factor structure of the RMBS and validity and reliability were also tested. Results The EFA suggested that a four‐factor solution was most interpretable, with few items cross‐loading, and there were common themes that unified the items in each factor, resulting in a 24‐item final measure. Cronbach's alpha confirmed the internal consistency, whereas bivariate correlations revealed the measure had good test‐retest reliability, discriminant validity, and convergent validity. Regression analyses established predictive validity of the RMBS for postpartum depressive symptoms. Conclusions The RMBS may be useful with clinical populations to identify maladaptive or rigid thoughts that could be a focus of intervention. This tool may also be used to guide conversation about motherhood expectations within any context where pregnant women present (e.g., prenatal care, social services), as well as potentially identifying women who are at risk for postpartum depression in clinical contexts.