New mothers in Western societies report being influenced by ideologies which suggest mothering comes naturally to women and is joyful and fulfilling. However, research reveals motherhood-related negative thoughts are common even among non-depressed new mothers, and it has been suggested experiencing such thoughts may be related to guilt and shame. This study updates and extends Hall and Wittkowski’s (2006) prevalence survey of motherhood-related negative thoughts by assessing new mothers’ perceptions of the social acceptability of negative thoughts, and by exploring relationships with guilt, shame and psychological distress. A cross-sectional survey design was used. A self-selected sample of non-clinical new mothers (N = 395) from the United Kingdom and Ireland completed online questionnaires including measures of the frequency and social acceptability of motherhood-related negative thoughts, shame and guilt proneness, depression and motherhood experience. Hierarchical regression analyses were used to explore relationships between variables. The frequency of negative thoughts was much higher than reported by Hall and Wittkowski. After controlling for demographic variables and social support, frequency of negative thoughts significantly predicted shame and guilt, whereas social acceptability of negative thoughts significantly predicted guilt. Negative thoughts, shame, guilt and motherhood experience relative to expectations significantly predicted depression score. These results suggest that negative thoughts are more common in early motherhood than previously reported, are considered socially unacceptable, and are related to guilt, shame and depression scores. The findings increase our understanding of postnatal distress in non-clinical populations. Future research should explore information and/or interventions aimed at “normalising” negative thoughts in early motherhood.