Why does the corpus luteum degenerate at the end of the ovarian cycle if fertilization does not occur, and what are the consequences for hormone levels?

When fertilization does not occur, LH blood levels fall, and because LH is the stimulus for luteal activity, the corpus luteum begins degenerating about 10 days after ovulation and eventually becomes a scar called the corpus albicans. As it degenerates, its output of progesterone and estrogens ends, so blood levels of these ovarian hormones drop sharply near the end of the cycle. This decline removes the negative feedback blockade of FSH and LH secretion, so a new ovarian cycle can begin.

The corpus luteum degenerates because, in the absence of pregnancy, its survival signal is lost. The evidence states that when fertilization does not occur, the stimulus for luteal activity ends when LH blood levels fall, and the corpus luteum degenerates. In about 10 days its hormonal output ends. Consequently, estrogen and progesterone levels drop sharply because the corpus luteum was the main source of these hormones during the luteal phase. The marked decline in ovarian hormones around days 26–28 ends their blockade of FSH and LH secretion, allowing gonadotropin levels to rise and the cycle to restart. In the uterine cycle, the fall in progesterone deprives the endometrium of support, causing spiral arteries to kink and spasm, endometrial cells to die, and menstruation to begin.