A randomised controlled trial evaluating the efficacy and mechanisms of online perinatal emotional skills groups compared to treatment as usual for women and birthing people with borderline personality disorder. Acronym: PROSPER: PeRinatal emotiOnal skillS Personality disordER

Background: Borderline Personality Disorder (BPD) is a mental disorder characterised by emotional instability, impaired functioning and an increased risk of suicide. People with BPD are at risk of adverse obstetric outcomes and their children are more likely to experience psychological difficulties. There is no clear evidence about how best to help this population during the perinatal period (covering pregnancy and the first 12 months after childbirth). Emotional Skills Groups (ESGs) teach people how to deal effectively with emotions and relationships. Research on non-perinatal populations indicates that generic ESGs may improve the mental health of adults with BPD. Perinatal ESG treatment consists of 12 online group sessions, targeting the emotional adjustment to becoming a parent. There is no robust evidence of efficacy of perinatal ESGs, and more broadly, it is unclear how ESGs work. Aims & Objectives: To establish the efficacy of perinatal ESGs (in addition to usual perinatal mental health care (UC)), compared with UC for patients with BPD in secondary mental health care. The embedded mechanistic study will determine whether any reductions in BPD severity are mediated by improvements in emotional regulation, mindfulness, distress tolerance and interpersonal sensitivity. A qualitative study will further explore mechanisms of action in depth. Methods: Two parallel group, multi-centre, individually randomised controlled trial (1:1), set in secondary care. Eligible patients will be aged =18 years, meeting DSM-5 criteria for BPD and either pregnant (from week 12 gestation) or with a baby less than 12 months old. Participants will be randomised to the intervention (12 group sessions of perinatal ESGs: weekly for 120 minutes) in addition to UC, or to continue with UC. The primary outcome will be global BPD symptom severity, assessed using the Zanarini Rating Scale for Borderline Personality Disorder-Self Report (ZAN-BPD SR) total score at 4 months post-randomisation. Secondary outcomes (all measured at 4, 8 and 12 months) will be emotional distress, mental wellbeing, social functioning, quality of life and infant bonding. Adverse neonatal outcomes and maternal sensitivity will also be recorded at 12 months. Intervention mediators will be measured during treatment delivery. A sample size of 216 will provide 90% power, at 2-sided 5% significance level, to detect an effect size of 0.5, allowing for 20% attrition at 4 months. This sample size generates 88%, 83%, and 77% power to detect a total indirect effect of 45%, 40%, and 35% respectively. Participant qualitative interviews will follow completion of primary efficacy outcomes (20 in intervention, 10 in UC), and qualitative interviews will also be conducted with trial therapists and their supervisors. Interviews will be audio recorded, transcribed and anonymised. Transcripts will be analysed thematically using a realist evaluation framework. Timelines for delivery: 43 months in total. Start 1st June 2025. Set-up phase 10 months. Internal pilot 7 months. If ‘go’ criteria met, recruit to 30th June 2027 and follow-up to 31st July 2028. 6 months for final analysis and dissemination of findings. Final report: 31st Dec 2028. Anticipated impact and dissemination: We will work with our PPI representatives and collaborators to maximise the dissemination of findings. The results will influence NICE guidelines and existing plans for increasing NHS perinatal mental health provision.