The COSI study is a 4-year NIHR HTA funded randomised controlled trial to explore the clinical and cost effectiveness of an 10-session, group-based intervention called Circle of Security Parenting (COS-P) for women in NHS perinatal mental health services (PMHS). COS-P is an intervention designed to help parents manage their difficult emotions and understand the impact of these on the mother-infant relationship. 

The trial will involve nine PMHS across England and will compare COS-P to treatment as usual with a sample of 369 participants. The primary aims are to identify clinical and cost-effectiveness, with maternal psychopathology (measured by the CORE-OM) as the primary outcome. Secondary outcomes assessed will include maternal sensitivity, emotional regulation skills, attachment security and infant development.

Research Question 

Is the Circle of Security-Parenting Intervention more effective than treatment as usual in improving symptoms of maternal mental illness and mother-infant bonding for mothers in NHS perinatal mental health services? 


Perinatal mental health problems are a significant public health concern, affecting up to 20% of new mothers and, when untreated, costing £8.1 billion per birth cohort. 72% of this cost is attributed to adverse impacts on the child, as a result of impaired mother-infant relationship. Perinatal mental health services (PMHS) provide multi-disciplinary support for women with mental health difficulties in pregnancy and the first post-natal year. However, these services are relatively new, and gaps in provision remain in group therapeutic approaches and mother-infant bonding therapies. COS-P is an intervention that can meet these needs, which shows preliminary efficacious findings and user acceptability. 


To conduct a multi-site national RCT comparing COS-P to treatment as usual (TAU) in improving maternal mental health and the quality of mother-infant relationships. Secondary objectives include exploring whether the intervention improves maternal sensitivity, emotion regulation skills, attachment security and has an impact on infant development. Additionally, cost-effectiveness and service-user and practitioner acceptability will be assessed. 


369 women across nine PMHS in England will be randomly allocated to TAU or COS-P. COS-P is a brief 10 session group parenting intervention that takes place over 10 weeks. Maternal mental health conditions will be assessed via CORE-OM at baseline, 3 months later (once the intervention has finished), and at 7- and 12- months after baseline. A suite of secondary outcome measures will also be assessed, as well as process evaluations. 


The primary analysis will aim to estimate the treatment policy estimand, and we will use the intention-to-treat principle including all participants who have at least one post-randomisation measure (3-month, 7-months, 12-months). This will use a mixed effects linear regression model to estimate the mean difference in CORE-OM between treatment group at 3-, 7- and 12- months. Supplementary analyses and analyses of secondary outcome measures will also take place. Qualitative data will be analysed via Thematic Analysis. An economic analysis will also take place that compares the costs and outcomes of COS-P to TAU.

Anticipated impact

This research will substantially add to the evidence base for psychological interventions that target maternal mental health problems and mother-infant relationship quality, areas which are highlighted in government policy. If effective, the research will lead to improved short- and long- term outcomes for mothers and children across a range of domains. Furthermore, COS-P has the potential to be used widely across all PMHS, given the established gaps in evidence based provision that it has the potential to fill, and as such this research can make a substantial contribution towards addressing this area of concern for the NHS. This intervention has the potential to reduce the costs of perinatal mental health problems to the NHS and wider society, through reduced cost to health, social, educational and criminal justice systems. 

Research team contacts

Primary Co-Investigators

Professor Peter Fonagy (Anna Freud National Centre for Children and Families),

Dr Camilla Rosan (Anna Freud National Centre for Children and Families) 


Dr Zoe Darwin (Reader, University of Huddersfield) 

Dr Victoria Cornelius (Reader in Medical Statistics, Imperial College of Science, Technology and Medicine)

Professor Steve Pilling (Professor of Clinical Psychology and Clinical Effectiveness, UCL)

Professor Pasco Fearon (Chair in Developmental Psychopathology, UCL)

Dr Elena Pizzo (Senior Research Fellow, UCL)

Lani Richards (PPI, Anna Freud National Centre for Children and Families)

Trial Team

Trial Manager: Kim Alyousefi-van Dijk (Anna Freud National Centre for Children and Families)

Clinical Trials Unit Operations Manager: Dr Daphne Babalis (Imperial College London)

COS-P Expert: Dr Nic Horley (West London NHS Trust)

NHS Lead – North England: Dr Ruth O’Shaughnnesy (North West Boroughs Healthcare NHS Foundation Trust) 

Research Assistant: Hannah Wright,

Research Assistant: Amy Shearson,

Research Assistant: Becky Scott,

Parent Participation Research Worker: Lani Richards,

General enquiries

Email address:

Tel: 07929752756

NHS recruitment sites

North Yorkshire & York

South West Yorkshire




Cheshire and Wirral

Mid-Mersey: Halton, Knowsley, St Helens, and Warrington

Liverpool and Sefton


Parent/Public Involvement

To support the COSI study, we are including an Experts by Experience (EBE) Panel. This will consist of women who have accessed PMHS across the nine sites involved in the study. This EBE panel will be led by our Parent Participation Research Worker - Lani Richards, who has previously completed a COS-P group. The EBE panel will be consulted regularly for their input throughout the study to ensure inclusivity and representation for the Participants of the study.

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