OBJECTIVE: In ART, progesterone (P4) level sometimes increases before HCG injection. Its impact on cycle outcome needs to be explored on large populations.DESIGN: Retrospective analysis of a prospective ART cohort.MATERIALS AND METHODS: All cycles between 2005 and 2008 in 2 ART labs related to one single endocrinology lab, with a P4 level on trigger day were included (n=8085). P4 was measured by chemiluminescence immunoassay, cycles were classified in 3 groups: <1.5, 1.5-1.9 and ≥2.0 ng/mL. We analyzed relationship between P4 level and women's age, stimulation results, cycle outcome, and pregnancy issue.Tabled 1Progesterone level at trigger day (ng/mL)<1.51.5-1.9≥ 2.0pcycles (%)86.08.65.4Women age35.3 ± 4.335.3 ± 4.334.8 ± 3.90.001InhB (pg/mL)62.2 ± 24.565.0 ± 21.366.0 ± 21.90.001E2 at trigger D (pg/mL)1833 ± 11732344 ± 12742610 ± 15390.001Collected oocytes8.9 ± 5.610.9 ± 6.211.8 ± 7.20.001Cleaved embryos4.7 ± 3.85.7 ± 4.26.1 ± 4.70.001PR/aspiration (%)26.524.417.90.001PR/transfer (%)30.925.820.90.001DR/pregnancy (%)78.273.369.40.03 Open table in a new tab CONCLUSION: High level of progesterone at trigger day reflects the quality of ovarian response. However, it may be detrimental on PR and DR, particularly in young women, probably because of a role on endometrial maturation. These results outline the importance of measuring P4 before triggering ovulation, especially among young women, in order to prevent early luteinization. OBJECTIVE: In ART, progesterone (P4) level sometimes increases before HCG injection. Its impact on cycle outcome needs to be explored on large populations. DESIGN: Retrospective analysis of a prospective ART cohort. MATERIALS AND METHODS: All cycles between 2005 and 2008 in 2 ART labs related to one single endocrinology lab, with a P4 level on trigger day were included (n=8085). P4 was measured by chemiluminescence immunoassay, cycles were classified in 3 groups: <1.5, 1.5-1.9 and ≥2.0 ng/mL. We analyzed relationship between P4 level and women's age, stimulation results, cycle outcome, and pregnancy issue. CONCLUSION: High level of progesterone at trigger day reflects the quality of ovarian response. However, it may be detrimental on PR and DR, particularly in young women, probably because of a role on endometrial maturation. These results outline the importance of measuring P4 before triggering ovulation, especially among young women, in order to prevent early luteinization.