 In a prospective observational study at a university teaching hospital, the mid-luteal peri-implantation endometrial thickness and volume, uterine artery pulsatility index PI and resistance index RI, endometrial and subendometrial vascularization index, flow index, and vascularization flow index, and serum estradiol and progesterone levels were compared between spontaneous and clomiphene citrate stimulated cycles in the same group of 50 patients with unexplained infertility. 