PURPOSE: Given the widespread contraceptive use of period-tracking applications, we aimed to quantify misclassification of biologically fertile days and associated pregnancy risk by calendar-based and BBT-supported period-tracking applications and compare it with an FDA-cleared and CE-marked digital contraceptive. MATERIALS AND METHODS: We conducted an observational analysis of 543,167 cycles from users of a fertility application. Biologically fertile days were identified using urinary luteinizing hormone tests. Outcomes included day-specific frequency of fertile days misclassified as safe, cycle-level misclassification, and mean predicted pregnancy risk per cycle for each method. Analyses were repeated in a subgroup of irregular cycles. RESULTS: Calendar-based tracking frequently misclassified fertile days as safe, with 67% of cycles containing ≥1 at-risk day and a predicted pregnancy risk of 22% (65% for irregular cycles). BBT-supported tracking reduced misclassification but remained associated with substantial risk (41% of cycles with ≥1 at-risk day; risk 9%). The digital contraceptive showed consistently low misclassification (3% of cycles with any at-risk day) and a predicted pregnancy risk of 0.5%. CONCLUSIONS: Both calendar-based and BBT-supported period-tracking applications often misclassify fertile days and should not be considered reliable tools for pregnancy prevention. Regulated digital contraceptives demonstrate substantially lower pregnancy risk than period-tracking apps.