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How Menstruation Works

Luteal Phase (approx. Day 16 to 28)

The corpus luteum degenerates.

The egg gets swept along by gradual wave-like motions of the finger-like projections in the walls of the Fallopian tube.

LH and FSH levels fall back to their low, steady levels.

Estrogen levels fall somewhat after the LH/FSH surge, but rebound due to continued secretion of estrogen and progesterone by the corpus luteum.

The uterine lining remains thick and ready to host a fertilized egg or growing embryo.

The corpus luteum shrinks and begins to die. It is programmed to die in 14 days unless it receives stimulation from human chorionic gonadotropin (hCG), a hormone secreted from a growing embryo.

  • If fertilization occurs, and the embryo implants in the lining of the uterus, hCG rescues the corpus luteum and it continues to secrete estrogen and progesterone throughout the pregnancy.
  • If fertilization does not occur, the corpus luteum dies at the end of the luteal phase. The unfertilized egg also dies and passes out of the uterus when menses begins. Estrogen and progesterone levels fall, menses starts and the uterine lining is shed. A new cycle begins.