Background Psychological distress is a common occurrence among women during the perinatal period. Maternal psychological distress (MPS) can also have a negative influence on neonatal outcomes such as infant health, child development or mother-child interaction. Hence, interventions to improve mental wellbeing during this period are vital. Mindfulness based intervention (MBI) has been found to be effective in reducing psychological distress. Delivery of MBI via the internet, making it accessible and inexpensive, is showing a promising positive effect in reducing psychological distress. A randomized control trial with sufficient power is required to confirm its positive effect among pregnant women. The positive effects of MBI have been found to be associated with heart rate variability (HRV) biofeedback; however, the efficacy of MBI on HRV has been rarely studied among pregnant women. Also, the potential association of HRV with MBI and psychological wellbeing needs further examination. This research aims to test the effectiveness of guided mobile-based perinatal mindfulness intervention (GMBPMI) among pregnant women experiencing psychological distress during the pre- and post-natal period, as well as examining the efficacy of GMBPMI on HRV. Method This study is a randomized controlled trial that follows a parallel design. Consenting pregnant women in their second trimester (between 12th and 20th week gestation) will be randomly assigned to an intervention group (GMBPMI) or a control group (psychoeducation). The intended sample size is 198, with 99 participants in each group. Three levels of outcomes will be measured at baseline, post intervention in both the intervention and control groups, and at 36-week gestation and five-week postpartum. The primary outcomes include maternal psychological stress, mindfulness and positive appraisal HRV. Secondary outcomes are psychological and physical wellbeing. Tertiary outcomes include obstetric and neonatal outcomes, and social support. Analyses will follow an intention-to-treat method and repeated measures MANOVA will be conducted to compare changes in primary and secondary outcomes. A series of mixed-effects models will be fitted to assess the mediation effects. Discussion This trial expects to increase understanding of GMBPMI on HRV and psychological wellbeing for pregnant women, with extended support in both pre-and post-natal periods. The study could also potentially provide evidence for delivery of cost-effective and accessible services to pregnant women. Trial registration ClinicalTrials.gov: NCT04876014, registered on 30 March 2021. Protocol Version 1.0., 10 May 2021.

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