Background: Mummatters is a web-based health tool that allows women to self-assess the symptoms of depression and the presence of psychosocial risk factors throughout pregnancy and the postnatal period. It aims to increase women's awareness of their own symptoms or risk factors and their knowledge of the available support options, to encourage engagement with these support options (as appropriate), and to facilitate communication about emotional health issues between women and their health care providers. Objective: The aim of this study is to report the uptake of mummatters; the sociodemographic and psychosocial risk profiles of a subsample of users; and the acceptability, credibility, perceived effect, and motivational appeal of the tool. The help-seeking behaviors of the subsample of users and barriers to help seeking were also examined. Methods: Mummatters was launched in November 2016. Women who completed the mummatters baseline assessment were invited to complete a web-based follow-up survey 1 month later. Results: A total of 2817 women downloaded and used mummatters between November 13, 2016, and May 22, 2018, and 140 women participated in the follow-up study. Approximately half of these women (51%; 72/140) were Whooley positive (possible depression), and 43% (60/140) had an elevated psychosocial risk score on the Antenatal Risk Questionnaire. Mummatters was rated favorably by pregnant and postnatal women in terms of its acceptability (94%-99%), credibility (93%-97%), appeal (78%-91%), and potential to affect a range of health behaviors specific to supporting emotional wellness during the perinatal period (78%-93%). Whooley-positive women were more likely to speak with their families than with a health care provider about their emotional health. Normalizing symptoms and stigma were key barriers to seeking help. Conclusions: Although mummatters was rated positively by consumers, only 53% (19/36) to 61% (22/36) of women with possible depression reported speaking to their health care providers about their emotional health. There was a trend for more prominent barriers to seeking help among postnatal women than among pregnant women. Future studies that investigate whether social barriers to seeking help are greater once a woman has an infant are warranted. Such barriers potentially place these women at greater risk of remaining untreated, as the demands on them are greater.