During which phase of the menstrual cycle does progesterone levels peak?

The luteal phase of the menstrual cycle is characterized by a peak in progesterone levels. After ovulation occurs, the ruptured follicle transforms into the corpus luteum, which releases progesterone. This hormone plays a crucial role in preparing the uterine lining for potential implantation of a fertilized egg, making it thicker and more vascular. If fertilization does not occur, the corpus luteum eventually degenerates, leading to a decline in progesterone levels and the onset of menstruation. This hormone's peak during the luteal phase is essential for regulating the menstrual cycle and supporting early pregnancy if fertilization occurs. Thus, the luteal phase is the correct answer for when progesterone levels are highest in the menstrual cycle.