AbstractWhen people stand still, they exhibit a phenomenon called postural sway, or spontaneous movement of the body's center of pressure, which is related to balance control. In general females show less sway than males, but this difference only begins to appear around puberty, pointing to different levels of sex hormones as one potential mechanism for sway sex differences. In this study, we followed cohorts of young females using oral contraceptives (n= 32) and not using oral contraceptives (n= 19), to investigate associations between estrogen availability and postural sway. All participants visited the lab four times over the putative 28‐day menstrual cycle. At each visit, we performed blood draws to measure plasma estrogen (estradiol) levels, and tests of postural sway using a force plate. During late follicular and mid‐luteal phase, estradiol levels were lower in participants using oral contraceptives (mean differences [95% CI], respectively: −231.33; [−800.44, 337.87]; −613.26; [−1333.60, 107.07] pmol/L; main effectp< 0.001), reflecting expected consequences of oral contraceptive use. Despite these differences, postural sway was not significantly different between participants who were using oral contraceptives and participants who were not (mean difference: 2.09 cm; 95% CI = [−1.05, 5.22];p= 0.132). Overall, we found no significant effects of the estimated menstrual cycle phase—or absolute levels of estradiol—on postural sway.