The association of endometrial thickness with live birth: a differential impact of agonist vs. antagonist ovarian stimulation protocols

ABSTRACT: To explore the relationship between endometrium, quality of transferred embryo, and their effects on the live birth outcome of IVF/ICSI-ET using different down-regulation protocol, we conducted a retrospective cohort in an academic reproductive medical center. A total of 13,585 participant's first fresh embryo transfer cycles, comprising 10,951 cycles using the GnRH-agonist regimen and 2,634 cycles using the GnRH-antagonist regimen were included in this study. We found that the first fresh embryo transfer cycles using the GnRH-agonist downregulation regimen achieved a significantly improved live birth rate (41.9%) when compared to the fresh cycles using GnRH-antagonist (30.4%). This was accompanied by a significantly thicker endometrium on the day of hCG administration. In the general population, increased endometrial thickness in the agonist group demonstrated a protective effect on the live birth. The cutoff value of endometrial thickness for live birth increased with the addition of poor-quality embryos in the double cleavage-stage embryo transfer cycles and the single blastocyst transfer cycles. The different choice of downregulation strategy affects the endometrium and the treatment outcome of IVF/ICSI-ET. LAY SUMMARY: The union of an embryo and the uterine lining is much like that of a seed and soil. The process of nurturing life resembles a seed taking root, sprouting, and bearing fruit. Different seeds require different conditions and soils. In assisted reproductive technology, the down-regulation protocol used can affect the state of the uterine lining. Embryos of varying quality also have different chances of successful implantation. Just as fertile soil supports a seed's growth, an endometrium of suitable thickness favors embryo implantation. When seed quality is less than ideal, richer soil may improve its chance to take root. Similarly, when available embryos are of lower quality, a thicker endometrial lining is associated with a higher live birth rate.