Is there evidence for preferential delivery of ovarian estradiol to the endometrium?

Objective: To determine the direction of delivery of E-2 in the female pelvis by assessing the ratio of endometrial to serum E-2 in women whose ovaries were stimulated to produce E-2 with women who received exogenous E-2. Design: Prospective comparative study. Setting: University-based ART program. Patient(s): Oocyte donors and recipients of donor oocytes. Intervention(s): Micronized E-2 administered by the oral or vaginal route and oocyte donation. Main Outcome Measure(s): Serum and endometrial levels of E-2. Result(s): Serum E-2 levels were significantly higher in women who underwent controlled ovarian hyperstimulation (COH) and women receiving exogenous E-2 by the vaginal route than in those who received oral E-2. Levels of E-2 in endometrial tissue were similar in women who underwent COH and those receiving oral E-2. Endometrial E-2 levels in women who underwent vaginal administration were significantly higher than those in the oral E-2 or COH groups. The ratio of endometrial to serum E-2 was highest in women who underwent vaginal E-2 and lowest in those undergoing COH. Conclusion(s): Vaginal administration of micronized E-2 results in preferential absorption of E-2 into the endometrium, consistent with a "uterine first pass" effect. Since endogenous E-2 produced the smallest ratio of E-2 between the endometrium and serum, E-2 produced by the ovaries is not preferentially delivered to the uterus. (Fertil Steril(R) 2001;75:1156-8. (C)2001 by American Society for Reproductive Medicine.).