This study employs a logit difference-in-differences model to estimate the extent to which raising the tobacco purchasing age to 21 (T-21) reduced the prevalence of smoking among mothers aged 18-20 both before and during their pregnancies. Using United States Vital Statistics data from 2012 to 2019, we estimate that counties which adopted T-21, whether individually or as part of a state-level policy, experienced small but statistically significant decreases in maternal smoking prior to and during their pregnancies. Our estimates also suggest that the decline in smoking during pregnancy is driven by fewer women smoking prior to pregnancy rather than increased quit rates during pregnancy. Finally, our estimation strategy also allows us to focus on the expansions in New York City and California, which are the locations with the largest number of impacted births in our sample. We estimate T-21 decreased maternal smoking both prior to and during pregnancy in California by up to 14% from baseline. Other studies which examined T-21 on a broader set of young adults have found larger reductions in smoking rates, suggesting that the subpopulation of young mothers who choose to smoke may be relatively less responsive to policies that raise the minimum purchase age of tobacco products.