Background <p>Postpartum women experience a range of physical, psychological, and social changes. Identifying and helping maladjusted women requires a comprehensive and efficient questionnaire. This mixed methods study aimed to examine the psychometric properties of the women’s postpartum adjustment questionnaire (WPPAQ) across a sample of Iranian postnatal women.</p> Materials and methods <p>This study employed the impact item method and exploratory factor analysis (EFA) to ascertain the psychometric properties and construct validity of the developed questionnaire. The primary questionnaire was developed based on a qualitative study and literature review. A principal components analysis (35 items) with Varimax rotation was undertaken in a cross-sectional study on 203 postnatal women.</p> Results <p>In the qualitative study and literature review, of the 34 items extracted, those with an impact item index of over 1.5, a content validity ratio of over 0.49, and a content validity index of over 0.79 were considered as valid. EFA reduced the list of the items to 30 with five factors capable of forecasting 57.19% of the variance. The Cronbach’s α coefficient for WPPAQ was 0.912.</p> Conclusions <p>Based on the results, WPPAQ designed in this study is valid and reliable for assessing women’s adjustment to postpartum changes and enabling healthcare providers to identify women who are not adequately adjusted to these changes.</p>