This study explores how professionals in specialist maternity care and social services work with pregnant women with substance use problems. It takes as its focus how support, control and interagency collaboration are enacted. Thus highlighting how organisational conditions and professional discretion shape interventions, approaches to care and ethical deliberations. These women often live in circumstances where substance use may coexist with mental health problems, trauma and social vulnerability. Support must thus be coordinated across services with different mandates, priorities and understandings. Based on qualitative interviews, the material was thematically analysed and interpreted through Michael Lipsky’s street-level bureaucracy and Michel Foucault’s disciplinary power. Findings show that support is organised differently across local contexts, shaped by geography, co-location and collaboration routines. Challenges in collaboration also arise when pregnancy’s timeframe clashes with organisational processes and the timelines of investigations/assessments. In encounters with women, alliance-building, flexibility and a non-judgemental approach are described as crucial for engagement and support. At the same time, intrusive measures, such as reporting concerns to child welfare services and, in some cases, compulsory care under the LVM Act, are described as ethically challenging because they may jeopardise the alliance. Professional discretion varies across contexts and strongly influences how interventions are carried out and how support and control are balanced. The study concludes that equity in support is influenced by local organisational conditions and the exercise of discretion. As such, it highlights the need for stable collaboration structures, clear routines, increased knowledge of each other’s mandates, and sufficient space for relational work to provide the best possible support for both the woman and the unborn child.