Unplanned pregnancy is a common problem among United States servicewomen. Variation among service branches in contraceptive education and access during initial training is associated with differences in contraceptive use and childbirth rates despite access to a uniform health benefit including no-cost reproductive healthcare and contraception. However, it is unclear whether changes in branch-specific contraceptive policies can influence reproductive outcomes among junior enlisted women in that service branch. To assess the longitudinal effect of contraceptive policy changes on contraception use and childbirth rates among military recruits. Secondary analysis was performed of insurance records from 70,852 servicewomen who started basic training between October 2013 and December 2016, assessing the longitudinal impact of a Navy policy change expanding contraceptive access during basic training implemented in January 2015, and a Marine Corps policy change restricting contraceptive access during basic training implemented in January 2016 on the following: contraception use (pills, patches, rings, injectable, implantable, and intrauterine) at 6 months, long-acting reversible contraception use at 6 months, and childbirth prior to 24 months after service entry. We used logistic and Cox regression models, adjusted for age group, to compare outcomes of women in the Navy and Marine Corps who started basic training before and after their service branch's policy change with outcomes among women in the Army and Air Force. Compared to the longitudinal difference observed among women attending Army or Air Force basic training, changing policies to increase contraceptive access during Navy basic training in January 2015 increased contraception use from 33.1% of sailors to 39.2% of sailors before and after the policy change (interaction term odds ratio, 1.31; 95% confidence interval, 1.22-1.41) and long-acting reversible contraception use 11.0% to 22.7% (odds ratio, 1.78; 95% confidence interval, 1.50-2.08). However, this policy change was not associated with a decline in childbirth rates among sailors (7.5% versus 6.1%) relative to the change among women in the Army and Air Force over the same time period (interaction term hazard ratio, 0.90; 95% confidence interval, 0.79-1.03). The January 2016 Marine Corps policy change decreased contraception use (29.6% to 24.4%; odds ratio, 0.78; 95% confidence interval, 0.70-0.88), long-acting reversible contraception use 14.6% to 7.3% (odds ratio, 0.39; 95% confidence interval, 0.31-0.48), and increased childbirth rates (8.0% to 9.6%; hazard ratio, 1.26; 95% confidence interval, 1.03-1.55) among Marines compared to outcomes in the Army and Air Force over the same time period. Basic training contraceptive policy influences contraception use among junior enlisted servicewomen. Implementing best practices across the military may increase contraception use and decrease childbirth rates among junior enlisted servicewomen. Copyright © 2020 Elsevier Inc. All rights reserved.