To elucidate if morning or evening start of exogenous progesterone (P4) five days before blastocyst embryo transfer (ET) impacts ongoing pregnancy rates (OPR) in artificial cycles.

Single-centre retrospective cohort study of 6493 artificial cycles for an ET (oestrogens and luteal phase support [LPS] with micronized vaginal progesterone [MVP] 400 mg/12 h), conducted at an infertility clinic, December 2018—July 2022. LPS was given from five days before ET. Until March 2021, LPS was started in the evening of day 0 of P4 exposure (“evening start”); since April 2021, LPS was started in the morning of day 0 (“morning start”).

Morning start of LPS (n = 2482 cycles); evening start (n = 3983 cycles). Morning or evening start did not exert any impact in OPR (46.9% vs. 46.3%, p = 0.682), as well as after adjusting for any potential confounders (aOR (95%CI): 1.00 (0.89–1.13); p = 0.996). Regarding serum P4 levels, no differences were found between morning (13.4 ± 5.8 ng/ml) and evening start of LPS (13.2 ± 6.4 ng/ml; p = 0.181). However, suboptimal serum P4 levels on the ET day (< 8.8 ng/ml) were registered in 16.6% (n = 411) vs. 19.8% (n = 788) of cycles with a morning and evening start, respectively (p = 0.001).

A 12-h difference in exogenous P4 initiation does not exert an impact on pregnancy outcomes in artificial cycles with MVP, as far as it is started five days before ET. Results from this study offer a great advantage in patient management, by being able to start LPS either in the morning or in the evening five days before blastocyst transfer.