Table 2

Barriers to uptake and retention mapped to COM-B and to potential behaviour change techniques

COM-B	Theme	10+Slimming World sessions	<10 Slimming World sessions	Behaviour change techniques that could help mitigate barriers
Motivation	Weight loss aspirations	Self-perceived as overweight	Some acknowledged overweight and understood why but viewed weight management as less of a priority	Provide information (leaflet/brief consultation) at outset to help influence knowledge and understanding regarding weight loss and consequences of action/no action
		Understood why gained weight	Others did not self-perceive as overweight	· Identity (framing/reframing; incompatible beliefs; identity associated with changed behaviour)
		Desired to lose weight after birth		Comparison of outcomes (pros and cons; credible source; comparative imagining of future outcomes)
				Goal setting (problem solving)
				Natural consequences (information about health consequences; salience of consequences; information about social and environmental consequences
Capability/ motivation	Beliefs and expectations about SW	SW and resulting weight loss helped change any pre-existing ideas about how to lose weight, esp re: role of exercise	Mistrustful of SW, in part as perceived as failing to prioritise exercise in line with pre-existing beliefs, seen as unsuitable for breastfeeding women, or felt gap in care for example, in relation to postnatal mental well-being	Leaflet/brief consultation (as above) to include:
				Shaping knowledge (Information about antecedents e.g. provide reliable info regarding weight gain/loss; reattribution, eg, elicit perceived causes of weight gain/how to lose weight and suggest alternatives)
				Goals and Planning (goal setting: behaviour for example, to make clear at the outset that the SW programme considers the role of exercise but focusses on diet; that 10+sessions are needed for it to be most effective; Goal setting: outcome – make clear at the outset that the outcome goals are focused on weight management, but that weight loss may have other positive outcomes such as improved sense of well-being/mental health.
				Comparison of outcomes (pros and cons: discuss pros and cons of exercise vs diet for weight loss)
Capability	Understanding and implementing SW	Understood dietary plan, found easy to follow, planned meals and used online resources	Did not appear to understand the dietary plan, only partial implementation, and experienced difficulties Making dietary adjustments	Development of a bespoke booklet to give to women at randomisation that summarises key elements of the programme (eg, some top tips/swaps; a sample 7-day menu plan); as well as optimising the following within the programme
		Considered plan unrestrictive, sustainable and compatible with postnatal lifestyle		Goals and planning (goal setting: behaviour; action planning)
		Positive benefits for them and their families		Feedback and monitoring (self-monitoring of behaviour, eg, complete food diary and get feedback from consultant)
				Shaping Knowledge (instruction on how to perform a behaviour)
				Repetition/Substitution (behaviour practice and rehearsal; graded tasks)
Opportunity	Social context/accessibility	Identified with group members/social bonds	Did not bond/socially identify with group	In the leaflet/brief consultation provided at randomisation, women can be reminded that they can change groups and consultants if desired; incorporate anticipation of impact of the programme on the family/partner into brief consultation (eg, how to discuss with partner; a section in the leaflet for partner/family to read)
		Positive relationships with group consultants, personalised support	Consultants unsympathetic to postnatal challenges	Social support (unspecified; practical; emotional)
		Supported by partners, childcare/sharing SW meals	Partner lack of support/partial adaptation to previous meals	Reward and threat (social incentive: explore whether their family/peers will congratulate them if they lose weight/attend SW sessions; restructure the social environment for example, by involving those who will be supportive of them attending SW, of dietary changes and weight loss)

COM-B, capability, opportunity, motivation and behaviour; SW, Slimming World.