Oestradiol concentrations during ovarian stimulation: association with embryo development and frozen embryo transfer...

Are serum oestradiol concentrations associated with the oestradiol/total oocyte ratio, and the oestradiol/metaphase II (MII) oocyte ratio with embryo morphokinetics and clinical outcomes in patients undergoing frozen-thawed embryo transfer after intracytoplasmic sperm injection (ICSI)?

A total of 16,760 embryos from 2584 women treated between March 2019 and December 2022 at a private university-affiliated IVF centre were analysed. Embryos were grouped into quartiles according to serum oestradiol concentrations on the day of oocyte maturation triggering, and morphokinetic and ICSI outcomes were compared. The oestradiol/MII and oestradiol/total oocyte ratios were also evaluated. Linear and non-linear spline regression models were applied to explore potential non-linear associations.

Spline analyses identified non-linear associations between oestradiol concentrations and embryological and clinical outcomes. Faster development and higher KIDScores were observed along the ascending portion of the spline curve (mid-range oestradiol; β = 2.0, P ≤ 0.003), whereas estimates shifted direction at the upper end of the distribution (β = -3.2; P < 0.001). Probabilities of implantation and pregnancy followed similar non-linear patterns, with higher model-predicted estimates up to the inflection point (implantation: β = 1147.0, P = 0.026; pregnancy: β = 107.6, P < 0.001) and attenuated estimates thereafter. Miscarriage rates were higher at the upper end of the oestradiol distribution (β = 37.6, P = 0.004).

Serum oestradiol concentrations showed continuous, non-linear associations with embryo morphokinetics and clinical outcomes, characterized by higher model-predicted estimates along the ascending portion of the spline curve and lower estimates beyond its upper inflection.