Two byproducts of in vitro fertilization, transvaginal ultrasonography and hormonal preparation of endometrial receptivity for oocyte donation, have been the source of great progress in endometrial physiology. In practical terms, transvaginal ultrasonography has permitted the assessment of endometrial thickness and more precise echogenicity, the latter being a potential reflector of the secretory changes induced by progesterone. Pulsed and color Doppler ultrasonography have further enhanced the diagnostic performance of transvaginal ultrasonography so that the vascular tone of uterine arteries can be correlated with the hormonal effects on the endometrium and the degree of endometrial receptivity. Because of the excellent pregnancy rates achieved with oocyte donation, estradiol and progesterone replacement regimens have become true experimental models for studying endometrial physiology and physiopathology. Although even extreme alterations in the estradiol-to-progesterone ratio appear to have little effect on endometrial morphology, recent studies suggest an extreme sensitivity of the endometrium to progesterone, particularly early in the luteal phase. The physiology of uterine bleeding and its disorders have been reconsidered in the light of these new findings. In the menstrual cycle, uterine bleeding is triggered by dropping progesterone levels, regardless of estradiol levels.