Perinatal depression poses substantial risks to both mothers and their offspring. Given its chronic and recurrent nature, developing effective prevention strategies is crucial. Internet-based cognitive–behavioural therapy (iCBT) has shown promise. However, the efficacy of specific CBT skills and the influence of individual differences remain unclear.

This protocol describes two harmonised multicentre, open-label, six-arm randomised controlled trials. Across both trials, a total of 2400 pregnant women between 10 and 20 weeks of gestation will be enrolled. After completing psychoeducation (PE), participants will be randomised to either the control condition (PE only) or one of five CBT programmes: behavioural activation (BA), assertion training, BA+cognitive restructuring, BA + problem solving or BA + behaviour therapy for insomnia. The objectives of the study are: (1) to ascertain that the iCBT approach is effective in perinatal depression, (2) to identify active CBT skills for perinatal women and (3) to examine interactions between these CBT skills and individuals’ baseline characteristics to find personalised and optimised therapy for individual women. The primary outcome is the point prevalence of depression at 1 month postpartum, defined as scoring of 9 or higher on the Edinburgh Postnatal Depression Scale.

The study has been approved by the Kyoto University Graduate School of Medicine Ethics Committee (C1710) and Nagoya City University Certified Review Board (2024A007). Anonymised study results will be presented at conferences and published by the investigators in peer-reviewed journals.

jRCTs042240162 (hospital-based, on-site trial) and jRCT1050250074 (nationwide online trial).