BACKGROUND: The aim of this study was to assess whether the cessation of progesterone (P) supplementation during early pregnancy after GnRH antagonist cycles is not inferior to its continuation in terms of pregnancy rates beyond 12 weeks of gestationMETHODS: There were 200 patients, with a positive β-hCG test (followed by a doubling in β-hCG levels 48 h later) after a fixed recombinant FSH (recFSH)/GnRH antagonist protocol for IVF/ICSI and a Day-3 fresh embryo transfer (ET), participated in this randomized controlled study. All patients received luteal support, with 200 mg vaginal P being administered three times daily for 14 days, beginning on the day of ET until the second β-hCG test, 16 days post-ET. In the control group (n = 100) the administration of P was continued until 7 weeks of gestation. In the study group (n = 100), vaginal P was discontinued on the 16th day post-ET RESULTS: The ongoing pregnancy rate beyond 12 weeks, the primary outcome measure, did not differ between the study and control groups (82 versus 73%, P = 0.175; difference 9%, 95% CI: -2.6 to 20.3). There were also no significant differences observed between the study and control group in terms of abortion before or after 7 weeks of gestation [(9 versus 12%, P = 0.645) and (8 versus 10%, P = 0.806), respectively]. The same was true for bleeding episodes (14 versus 19%, P = 0.446).CONCLUSIONS: After recFSH/GnRH antagonist cycles, the withdrawal of P supplementation in early pregnancy, with normally increasing β-hCG levels on the 16th day post-ET, had no significant clinical impact in terms of ongoing pregnancy rates beyond 12 weeks.