State-level marijuana legalization has unintended consequences, including its effect on fertility. Marijuana use is associated with behaviors that increase fertility as well as physical changes that lower fertility. In this paper, I use a difference-in-differences design that exploits variation in medical and recreational marijuana legalization across states and over time to study the effects of marijuana legalization on fertility. This paper is the first to study the effects of recreational marijuana legalization on fertility. I find that legalizing recreational marijuana decreases a state’s birth rate by an average of 2.78% while increasing the probability that an individual is sexually active by 3.6 percentage points. [emphasis added by TC] Together, my findings show that the physical effects of marijuana use have the dominant effect on fertility. By contrast with the existing literature, I find that medical marijuana legalization does not affect the birth rate, although it increases the frequency of sexual activity by 1.6 sexual encounters per month. Neither type of marijuana legalization affects male gonorrhea cases or the probability of having sex with a stranger.