BACKGROUND: We investigated the acceptability and feasibility of a new brief intervention for maternal prenatal anxiety within maternity services in London and Exeter, UK. METHODS: One hundred fourteen pregnant individuals attending their 12-week scan at a prenatal clinic with elevated symptoms of anxiety (GAD-7 score of ≥7) were randomly assigned to either the ACORN intervention + Treatment as usual (TAU) (n = 57) or to usual care only (n = 57). The ACORN intervention consisted of 3 2-h group sessions, led by a midwife and psychological therapist, for pregnant individuals and their partners. The intervention included psychoeducation about anxiety, strategies for problem-sovling and tolerating uncertainty during pregnancy, including communicating about these with others, and mindfulness exercises. RESULTS: Engagement rates with ACORN met or exceeded those in primary care services in England. In the intervention arm, 77% (n = 44) of participants attended at least one session, 51% (n = 29) were adherent, defined as attending two or more sessions. Feedback was positive, and participants in the ACORN treatment group demonstrated evidence of a larger drop in their levels of anxiety than the participants in the TAU-only group (Cohen's d = 0.42). CONCLUSION: The ACORN intervention was acceptable to pregnant individuals and their partners and resulted in reductions in anxiety. With further evaluation in a larger-scale trial with child outcomes, there is significant potential for large scale public health benefit.