Objectives: To determine rates of luteal phase pregnancy (LPP) in young women initiating subdermal implants on any day of the menstrual cycle. Study design: We assessed a retrospective cohort of young women receiving contraceptive implants at an adolescent Title-X clinic. Patients with negative pregnancy tests were eligible for same-day insertion, regardless of cycle day, contraceptive use, or last intercourse. We computed LPP rates for those within manufacturer guidelines for insertion (<= 5 days of menstrual onset or days post-discontinuing hormonal contraception) and outside these guidelines. We reviewed medical records for last menstrual period (LMP), current hormonal contraception, emergency contraception (EC) provision, and pregnancy tests weeks post-implant placement, or later evidence of pregnancy. For patients with positive pregnancy tests or reports, we used standard obstetrical dating (LMP and ultrasound) to determine if conception occurred 2 weeks of implant placement. Results: Of 3180 insertions, 1868 (58.8%) were outside recommended guidelines. Women with insertions within-guidelines were older (20.2 vs. 19.3 years; p<0.001) and more likely to be white (40.4% vs. 29.5%; OR=1.6, 95% CI: 1.4-1.9). Definitive pregnancy data was documented for 1726 patients: 660 (50.3%) in the within guidelines group, and 1066 (57.0%) in the outside guidelines group. Rates of LPP were 0.3% (2/660; 95% CI=0.0-1.1%) in the within guidelines group and 0.9% (10/1066; 95% CI=0.5-1.7%) in the outside guidelines group. Conclusion: The risk of LPP following any-cycle-day insertion of contraceptive implants with negative pregnancy testing is low, regardless of menstrual cycle timing, recent contraceptive use or use of EC. (C) 2017 Elsevier Inc. All rights reserved.