BACKGROUND: Physical activity (PA) tends to decline during pregnancy, and remains low in the early postpartum period, despite the known physical and psychological benefits. This study aimed to explore: (1) women's experiences of PA during pregnancy and following birth; and (2) decision-making processes related to PA during this time. METHODS: Semi-structured telephone interviews were conducted with 16 women who were either pregnant or had recently given birth. Interviews were transcribed verbatim and analysed using thematic analysis. RESULTS: There were two over-arching themes: (1) ownership of body, which consisted of the sub-themes: others try to take ownership, important to maintain fitness into pregnancy and motherhood, expectations of PA, and pressure to conform; and (2) unknown territory, which consisted of the sub-themes: engaging in PA with caution, and unclear advice. Decision-making about PA during pregnancy was influenced by: pressure from others who felt responsible to protect the woman from coming to harm; pressure from social media to lose weight postpartum; participant's beliefs about the benefits of maintaining fitness and participants expectations of how active they thought they would be able to be during pregnancy. Participants felt that pregnancy was an 'unknown territory' in terms of the unfamiliar feelings in their body and the inability to continually monitor their baby for reassurance of baby's health. Advice received from midwives was often lacking, or not tailored to the individual. Advice from friends and family was often regarded as incorrect, but still caused doubt and fear of PA during pregnancy. CONCLUSIONS: These findings contribute to the understanding of women's experiences of PA during pregnancy and post-partum, and their decision-making processes about PA during pregnancy. Developing accurate and tailored advice as part of midwifery care, that considers the physical and psychological aspects of engaging in PA during pregnancy, will help to ensure that women are supported to make informed decisions about their PA behaviour.