Maternal cortisol plays an important role in foetal development and is often used to measure stress. In addition to stress, prenatal and post‐partum women experience a mix of positive and negative emotions. However, few studies have examined the relationship between cortisol and maternal psychological health or how these relationships change from the prenatal to the post‐partum periods. The purpose of this study was to examine the relationships between maternal cortisol and stress, happiness, and depression components of psychological well‐being, in late pregnancy and early post‐partum. The study used a repeated measure design with 41 women; 21 women completed all data collections. As expected, maternal salivary cortisol increased from awakening to 30 minutes after awakening during pregnancy. Levels of salivary cortisol were lower at post‐partum. Stress, happiness and depression were significantly correlated at both prenatal and post‐partum measurements. Prenatal cortisol awakening response was correlated with happiness. Maternal psychological well‐being and cortisol did not differ by parity, race or employment. Maternal cortisol could potentially serve as a biochemical marker of maternal psychological well‐being. Interventions to decrease maternal stress may promote maternal psychological well‐being. Longitudinal studies with larger sample and diverse ethnicities may increase our understanding of the role of cortisol in foetal and maternal health. Copyright © 2009 John Wiley & Sons, Ltd.