Of the several follicles that commence growth, one becomes more prominent by a surge in rapid growth and activity. As the Graafian follicle grows, it secretes increasing amounts of estrogen. The remainder of the follicles regress. Day 21 or 0. The high estrogen level in the blood triggers a release of LH near the end of heat.
Progesterone again becomes the dominant hormone.
It must be noted that the timing given for the preceding events is only approximate and differs for different cycle lengths. The discussion of events occurring during the previous cycle was based on a full cycle in which pregnancy does not occur. If the egg is fertilized and begins development in the uterus, the corpus luteum does not regress but continues to function by secreting progesterone. No follicles develop to maturity and heat does not occur. Progesterone keeps the uterus quiet and thus provides the most favorable conditions for the developing fetus.