The serum oestradiol/progesterone ratio on the day of OPU + 7, but not the day of OPU + 5, affects the rates of live birth in fresh blastocyst embryo transfer cycles

Background In an in vitro fertilization (IVF) cycle, the embryo ends its wandering time and begins the process of implantation into the uterine cavity on the seventh day after oocyte pick-up (OPU + 7), which is closer than OPU + 5 to the time of nidation. Therefore, measuring the oestradiol (E2)/progesterone (P) ratio on OPU + 7 may be helpful for predicting pregnancy outcomes. Methods This is a retrospective cohort study of 2,257 women undergoing a follicular-phase depot gonadotropin-releasing hormone agonist (GnRH-a) protocol for in vitro fertilization /intracytoplasmic sperm injection (IVF/ICSI) treatment and fresh blastocyst embryo transfer cycles at a university-affiliated fertility center between January 2016 and April 2021. First, 2,257 women were split into two groups based on clinical pregnancy for analyzing the levels of E 2 and P and the E 2 /P ratio on the day of OPU + 2, OPU + 5 and OPU + 7. And then 2,257 cycles were stratified into three groups based on E 2 /P ratio tertiles on OPU + 7: the low group (1.3–15.7 pg/ng), middle group (15.7–28.8 pg/ng), and high group (28.8–487.2 pg/ng). The threshold effect of the E 2 /P ratio on OPU + 7 on live birth was investigated using a two-piecewise linear regression model and a smoothing function curve. Results The level of P in the clinical pregnancy group were lower than that in the nonclinical pregnancy group on both OPU + 2 and OPU + 7 (201.9 ± 71.6 ng/ml vs 213.1 ± 77.6 ng/ml, 89.5 ± 88.5 ng/ml vs 99.5 ± 94.9 ng/ml, P < 0.05). The E 2 /P ratio in the clinical pregnancy group were higher than that in the nonclinical pregnancy group on both OPU + 2 and OPU + 7 (8.4 ± 6.5 pg/ng vs 8.0 ± 6.8 pg/ng, 32.3 ± 38.5 pg/ng vs 25.2 ± 31.0 pg/ng, P < 0.01). The E 2 /P ratio on OPU + 7 was positively associated with positive hCG (adjusted OR = 1.01; 95% CI, 1.01–1.02; P < 0.0001), clinical pregnancy (adjusted OR = 1.01; 95% CI, 1.00–1.01; P = 0.0067) and live birth (adjusted OR = 1.01; 95% CI, 1.00–1.01; P < 0.001), and a nonlinear correlation was observed between the E 2 /P ratio and LBR on OPU + 7. Conclusions A higher E 2 /P ratio is associated with a higher LBR, but the E 2 /P ratio should be maintained within a suitable range.