Objective: We assessed (1): the factorial and convergent validity of the Portuguese version of the Stress Appraisal Measure (SAM); (2) maternal primary and secondary appraisals of the birth of a child; and (3) the influence of appraisals on stress. Background: Parental stress is common during transition to parenthood and may be detrimental for parents and infants. Following Lazarus and Folkman’s transactional model, the degree of stress is influenced by cognitive appraisals of an event, which are assessed by the SAM. Methods: This was an observational, cross-sectional study. Two hundred and forty-five women with a child of up to 24 months of age filled out several self-report measures, in an online platform. Results: The SAM factorial structure included three primary (Threat, Centrality, and Challenge) and two secondary (Controllable-by-Self and Controllable-by-Others) appraisals. All subscales had adequate internal consistency. The birth of a child was mainly perceived as a challenge, and seldom appraised as a threat. Threat and centrality appraisals positively predicted stress. Conclusion: Assessing maternal appraisals of the birth of a child is relevant to foster adaptation to parenthood. The SAM is an adequate assessment tool. As cognitive appraisals are modifiable, women with higher threat appraisals should be targeted for psychological interventions.