['To investigate the relation between the development of endometrial pinopodes and the serum concentration of hormones and the distribution of estrogen receptor-alpha, estrogen receptor-beta, progesterone receptor A, and progesterone receptor B.', 'Prospective clinical study.', 'Hospital-based unit of reproductive health and university-affiliated reproductive research laboratories.', 'Twenty-seven healthy fertile women with normal menstrual cycles.', 'Urine and blood sampling for hormone measurement, vaginal ultrasonography, and endometrial biopsy.', 'Appearance of the endometrium on light microscopy, pinopode formation, serum levels of luteinizing hormone (LH) and follicle stimulating hormone (FSH), and expression of progesterone receptors A and B and estrogen receptors alpha and beta.', 'Pinopode formation and regression were closely associated with increases and decreases, respectively, in serum progesterone concentration. At pinopode development, levels progesterone receptors A and B in the glandular and luminal epithelial cells decreased; this effect was mainly dependent on the absence of progesterone receptor B. Serum estrogen levels and levels of estrogen receptor alpha and beta did not correlate with pinopode formation.'] Is formation of pinopodes in human endometrium associated with the concentrations of progesterone and progesterone receptors?