During the natural reproductive cycle, this process takes place in two stages. During the endometrium's proliferative phase, which takes place during the first 14 days of a 28 day cycle, estradiol is produced by the growing follicle whereby the thin endometrium remaining after the menstrual cycle proliferates to a width of between 7 to 14 mm on average, attains a triphasic appearance on ultrasound and acquires receptors for progesterone. After ovulation, the ovarian follicle transforms to a corpus luteum which produces progesterone. At this stage, exposure to progesterone transforms the endometrium to one which is receptive to embryo implantation. There is a minimum level of progesterone and a minimum time of exposure which is necessary to transform the endometrium to a receptive one while prolonged progesterone exposure renders the endometrium hostile to implantation. The boundaries of this time zone are referred to as the window of implantation. The length of time of this window is unknown.