Cannabis use in the United States has been steadily increasing. Much more is known about cannabis users in states where recreational cannabis is legal, compared to states yet to legalize. Further, there is little information known about reasons for cannabis use during critical maternal and child health periods. To address this gap, we examined cannabis use among New Hampshire women during the preconception, prenatal, and postpartum periods, and explored reasons for use. We analyzed data from the 2016–2017 Pregnancy Risk Assessment Monitoring System (PRAMS) in the Spring of 2020. For women delivering a live-born infant, PRAMS collects behavior and experience data before, during, and after pregnancy. Using data from New Hampshire, we estimated the weighted prevalence of self-reported cannabis use during the preconception, prenatal, and postpartum periods, and reasons for use during each period. Of the 1147 women included in the analysis, 16.8% (unweighted n = 186) reported preconception cannabis use, 5.5% (unweighted n = 68) reported prenatal cannabis use, and 6.6% (unweighted n = 73) reported postpartum cannabis use. About one-fifth (20.3%) of women who reported cannabis use reported use during all three periods. The top reason for cannabis use during preconception and postpartum was for stress or anxiety relief (64.88% and 73.06, respectively), and during the prenatal period was for its’ antiemetic properties (84.1%). Our findings support that women’s healthcare providers should screen patients for cannabis use, addressing underlying reasons (e.g., mental health issues, nausea/vomiting) for use as part of the screening process.