Background Adherence to physical activity (PA) recommendations during pregnancy is low. A common reason for low adherence is concern of harm to mother and/or baby. The Necessity-Concerns Framework (NCF), is a well-established framework in medicine adherence, however it has not been used to explore adherence to antenatal PA. This study aims to explore (1) what influences pregnant women's PA in the context of the NCF; and (2) if the NCF is an appropriate framework to understand antenatal PA engagement. Methods Semi-structured interviews were conducted with 18 pregnant women in the UK and Ireland (mean gestation 27 weeks). Interviews explored beliefs, experiences, perceived necessities and concerns about PA. Interviews were transcribed verbatim and analysed using thematic framework analysis. Results Five themes were identified as influential to antenatal PA: (1) Perceived benefits and necessity of PA, (2) Concerns regarding antenatal PA, (3) Balancing the necessity and concern, (4) Barriers to antenatal PA, (5) Facilitators of antenatal PA. Women described a number of perceived necessities and concerns regarding antenatal PA. These necessities and concerns were described as being consciously balanced, supporting the NCF. However, a number of additional influences (for example, feelings of nausea and lack of advice and knowledge) seemed to impact antenatal PA engagement before women could consider their perceived necessities and concerns around antenatal PA. Conclusions The Necessity Concerns Framework is a useful framework to help explain how and why women engage with antenatal PA, more specifically why women do and do not engage in antenatal PA at different times during their pregnancy. However, there are a number of other interpersonal and intrapersonal influences on antenatal PA (e.g. physical symptoms, motivation and time), suggesting the NCF alone may be too simplistic to understand and influence complex behaviour such as antenatal PA
