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The hypothesis was tested that conception cycles (CC) resulting from IVF can be distinguished from non-conception cycles (NC) by differences in corpora lutea function that are detectable at the earliest stage of embryo implantation. Luteal oestradiol secretion was analysed retrospectively in 409 ovarian stimulation cycles of 296 patients from the day of embryo transfer until 14 days after embryo transfer (ET+14) in IVF/intracytoplasmic sperm injection (ICSI) cycles. Human chorionic gonadotrophin (HCG) was administered in 330 of 409 cycles in addition to vaginal progesterone in all cycles. Differences in serum oestradiol concentrations between CC and NC increased from day ET+1 onward and became statistically significant on days ET+4 through ET+14, with higher oestradiol concentrations in CC compared with NC. Even though exogenous HCG administration prevented the fall in luteal oestradiol concentrations after ET+4 both in CC and NC, increasing differences in oestradiol concentrations between CC and NC after embryo transfer were observed in both groups of HCG-supplemented and non-supplemented cycles. It is concluded that luteal oestradiol secretion is affected at the earliest stage of embryo implantation. The putative early signal to the corpus luteum associated with embryo attachment and early implantation appears to be superimposed onto the effect of exogenous luteal HCG administration and is clearly distinguishable as early as 4 days after embryo transfer in conception cycles. [ABSTRACT FROM AUTHOR]