After the egg is released, the luteal phase of the menstrual cycle begins. The released egg has about 24 hours to get fertilized by waiting sperm, and this usually occurs just as the egg enters the fallopian tube. If you had had sex on the days of the LH surge, sperm should be there, waiting to greet the ovulated egg.
In the meantime, the LH hormone causes the ruptured follicle to become what is known as a corpus luteum. The corpus luteum's job is to keep releasing estrogen and to release a new hormone, progesterone. Estrogen encourages the lining of the uterus to keep growing, while the hormone progesterone helps the lining of the uterus become receptive to the fertilized egg.
Progesterone is responsible for those imaginary pregnancy symptoms that torture many of us during the two week wait. The hormone progesterone also causes a slight increase in your body temperature, which is what causes the temperature shift you see if you're charting your basal body temperature.
The corpus luteum's life is short, however. If an egg is fertilized, the embryo will release the hormone hCG, or human chorionic gonadotropin. hCG is very similar to the hormone LH, and it keeps the corpus luteum alive, producing more estrogen and progesterone to maintain the pregnancy.
But, if the egg did not become fertilized, then the corpus luteum begins to disintegrate about three days before you get your period. The estrogen and progesterone hormone levels drop, causing the endometrium to break down and eventually lead to menstruation.
While we are crying about another unsuccessful month, however, our bodies are wasting no time with pity parties. The day your period begins is the day that the hypothalamus begins releasing GnRH once again, starting off another marathon for the next group of waiting follicles.