Abstract: Importance: Maternal nutritional status, including vitamin B12 and/or folate deficiency, has long been recognized as a risk factor for neurodevelopmental disorders and some medications are known to deplete folate and/or vitamin B12 levels. Objective: We examined whether maternal use of folate and/or vitamin B12–depleting medications during pregnancy is associated with increased odds of autism spectrum disorder (ASD) in their children. Design, Setting, and Participants: A retrospective case-control study was performed using the Military Healthcare System (MHS) database. Cases were identified by International Classification of Diseases, Ninth Revision Codes for ASD and were matched 3:1 with controls on sex, date of birth, and enrollment timeframe. Conditional logistic regression analyses were used to calculate the unadjusted and maternal risk factor-adjusted odds ratios, along with 95% confidence intervals, for the risk of ASD associated with each medication. Exposure: The use (dispensed) of folate and/or vitamin B12-depleting medications during pregnancy. Main Outcome: Autism Spectrum disorder diagnosis in their children. Results: The odds of having a child with ASD were significantly higher for mothers dispensed the following medications during pregnancy: proton pump inhibitors (adjusted (aOR)=1.26, 95% [CI 1.08-1.48]), quinolones (aOR=1.23, 95% CI [1.02-1.49]), valproate products (aOR=2.47, 95% CI [1.23-4.95]), phenobarbital (aOR=2.62, 95% CI [1.13-6.07]) and metformin (aOR=1.64, 95% CI [1..29-2.09]). Conclusion and Relevance: We report modest to strong associations between prenatal exposure to some medications and the risk of ASD in the offspring. Some of the associations may be due to residual confounding; therefore, these findings should be interpreted with care. Treatment decisions in pregnancy should continue to weigh maternal benefits and fetal risks on an individual basis. If future studies confirm these findings, this may be an opportunity to inform public health and clinical practices, thus possibly decreasing the incidence of ASD in a subset of children.