Final answer:
The degeneration of the corpus luteum is triggered by the increased estrogen production by the ovaries when the placenta is ready to take over during pregnancy.
Step-by-step explanation:
The degeneration of the corpus luteum once the placenta is ready to take over during pregnancy is triggered by the increased estrogen production by the ovaries. As the placenta develops and the corpus luteum degenerates, the placenta gradually takes over as the endocrine organ of pregnancy. The placenta converts weak androgens secreted by the maternal and fetal adrenal glands to estrogens, which are necessary for pregnancy to progress.