Final answer:
Progesterone is highest and 17-β-estradiol is lowest during the luteal and secretory phases of the estrous cycle, which occur after ovulation when the corpus luteum is active in secreting progesterone.
Step-by-step explanation:
During the estrous cycle, progesterone levels peak during the luteal phase of the ovarian cycle and the corresponding secretory phase of the uterine cycle. In contrast, the estrogen hormone, such as 17-β-estradiol, is at its lowest during the same phase. After ovulation, the ruptured follicle forms the corpus luteum which secretes high levels of progesterone, initiating the secretory phase in the uterus. The progesterone is crucial for facilitating the regrowth of the endometrial lining and inhibits the release of Follicle-stimulating hormone (FSH) and Luteinizing hormone (LH) to prevent further egg development. Meanwhile, estrogen levels stabilize at this time.
Without fertilization, the corpus luteum degenerates, ceasing its production of progesterone, which leads to the menstruation phase and the restart of the cycle with the growth of new follicles that secrete estrogen. Therefore, the highest concentration of progesterone and the lowest of estrogen occur during the latter half of the cycle.