Feedback

• Follicular Phase (Days 1-14):

  • Early-Mid Follicular: Low estrogen levels exert negative feedback on the hypothalamus and pituitary, inhibiting FSH and LH release to ensure the development of a single dominant follicle. Inhibin B, produced by developing follicles, also selectively inhibits FSH secretion.
  • Late Follicular (Pre-Ovulatory): The dominant follicle produces a large amount of estrogen. Once estrogen levels are high and sustained (>200 pg/mL for ~50 hours), it paradoxically exerts positive feedback on the pituitary and hypothalamus.
  • Result: This positive feedback leads to a massive LH surge (and a smaller FSH surge), which triggers ovulation within 24-48 hours.

• Luteal Phase (Days 14-28):

  • Corpus Luteum Formation: After ovulation, the ruptured follicle becomes the corpus luteum, which secretes high levels of progesterone and moderate levels of estrogen.
  • Negative Feedback: The combination of high progesterone and estrogen exerts strong negative feedback on the hypothalamus (decreasing GnRH pulse frequency) and pituitary, suppressing both FSH and LH release. This prevents the development of new follicles during this phase.
  • Result: If no pregnancy occurs, the corpus luteum degenerates, leading to a sharp drop in progesterone and estrogen. This decline removes the negative feedback, allowing GnRH/FSH/LH to rise and initiate a new follicular phase.