To examine the relationship between endometrial histological maturation and reproductive hormones, we studied 11 fertile women, aged 18 -37 yr. All participants had had at least 1 previous pregnancy and cycled regularly, every 25-35 days. Women collected daily, first morning voided urine for measurement of estradiol and progesterone metabolite excretion, estrone conjugates (E1c), and pregnanediol glucuronide (Pdg), respectively, throughout the cycle of study. Hormones were normalized for creatinine. Between 7-9 days after home detection of a LH surge (Sure Step), participants underwent an endometrial biopsy using a small bore (Pipelle) catheter. Tissue was prepared for histological and biochemical analyses. The histological analysis is reported herein. Endometrium was dated by 3 authors (N.S., D.H., and S.P.), all of whom were blinded to the participant's identity or timing of biopsy within her cycle. Final dating was agreed upon based upon the method of Noyes et al. E1c and Pdg were integrated throughout the cycle using the trapezoidal rule, and correlations were sought between deviation from expected histology (based upon urinary hormones and LH surge) and integrated hormone values.E1c varied over a 2-fold range in these normal women, from 1196 -2040 ng/cycle. Pdg excretion was much more variable, ranging from 22-119 g/cycle. No relationship could be found between histological lagging of endometrial maturation and lower excretion of E1c. A moderate correlation was observed (Spearman's r ϭ 0.6; P Ͻ 0.05) between degree of histological maturation and integrated Pdg. Of two women with evidence of a disparity between gland and stromal development (glands lagging behind stroma by Ͼ2 days), one excreted 24 g Pdg/cycle, the next to lowest value.We conclude that normal fertile women experience a wide range of hormone concentrations in the face of normal endometrial maturation. Progesterone appears to exert a dose-related effect on endometrial maturation, and the techniques we used, although relatively crude clinical measures, appeared to be sufficient to detect this relationship. (J Clin Endocrinol Metab 85: [4207][4208][4209][4210][4211] 2000) E NDOMETRIAL HISTOLOGY varies across the normal menstrual cycle and is due to stimulation by estradiol and progesterone. Estradiol stimulation results in proliferation of the basalis and functionalis, whereas progesterone promotes glandular development and secretion and is believed to initiate the changes necessary for implantation. Progesterone appears essential for the process of normal endometrial maturation, and a window of implantation, approximately 6 -8 days after ovulation, has been defined in the human (1-3).The clinical concept of luteal phase insufficiency presumes that subthreshold progesterone stimulation of the uterus can cause failure of endometrial development. Moreover, clinical criteria for endometrial inadequacy have been defined, albeit arbitrarily, by inspection of histological specimens of endometrial biopsies taken from the mid to late luteal p...