Background Cannabis is the most commonly used drug during pregnancy, excluding alcohol and tobacco, in the United States. Cannabis consumption during pregnancy is increasing along with greater legal and social acceptance. Methods We conducted a qualitative content analysis of 23 in-depth interviews with pregnant and lactating people in Massachusetts, a state that legalized cannabis for adult use in 2016. Our aim was to explore how policy constrains or facilitates people's ability to make informed decisions about cannabis use during pregnancy and lactation. Our analysis was conducted using an ecosocial approach, recognizing that the implementation and interpretation of cannabis policy can be understood at multiple levels, which interact with each other and shape the health and experiences of individuals. Additionally, this analysis was informed by a harm reduction approach in which we acknowledge the complexity surrounding cannabis use during pregnancy and lactation, while attempting to identify ways to reduce potentially harmful consequences. Results Findings revealed that, despite the legal status of cannabis, there continues to be a lack of clarity for pregnant and lactating people regarding the legal implications of cannabis use. Inconsistent state and institutional policies about drug testing of mothers and newborns leave a cloud of fear hanging over the experiences of people who use cannabis and inhibit their ability to obtain expert advice from healthcare providers. Conclusion Decision makers in public and institutional policy should work to clarify and update policies regarding substance use during pregnancy following legalization of a new substance, and ensure that pregnant and lactating people are afforded the same legal protections as the general population.