Assessing the early luteal phase in in vitro fertilization cycles: relationships between plasma steroids, endometrial receptors, and endometrial histology.

The corpus luteum-endometrial unit was investigated in in vitro fertilization (IVF) cycles using endocrine, morphologic, and biochemical measurements on the day normally scheduled for embryo transfer (day 16), in 12 stimulated and 4 natural cycles. Advanced endometrial histologic maturity was recorded in 9 of the 12 stimulated cycles. No in-phase endometria were seen when the preovulatory plasma estradiol (E2) was greater than 500 pg/ml or the day 16 plasma progesterone (P) greater than 10 ng/ml in natural or stimulated cycles. Significant negative correlations were noted between both preovulatory E2 and day 16 P and the concentration of cytosolic progesterone receptor (PRc). Advanced endometrial maturity tended to be associated with low concentrations of PRc. Regardless of endometrial maturity, the natural cycle was characterized by low cytosolic E2 receptors (ERc) and high PRc, whereas the concentration of both receptors was usually greatly reduced in stimulated cycles. It is concluded that the advanced endometrial maturation observed in stimulated IVF cycles is a consequence of the production of supraphysiologic levels of sex steroids by the corpus luteum that cause profound modifications of endometrial receptor dynamics.