Key Points Question Did rates of cannabis use during early pregnancy change after recreational cannabis legalization (RCL) in California, and did trends differ in jurisdictions that allowed vs banned adult-use retailers? Findings This time-series study of 300 993 pregnancies universally screened for prenatal cannabis use at entrance to prenatal care in Northern California from 2012 to 2019, there was an increase in self-reported and biochemically verified prenatal cannabis use after RCL implementation in 2018. The increase was observed only among those living in jurisdictions allowing adult-use cannabis retailers. Meaning The implementation of RCL in California was associated with an increase in prenatal cannabis use, driven by individuals living...