Final answer:
The menstrual cycle relies on hormone levels to regulate the fate of the endometrium. If fertilization occurs, progesterone maintains the uterine lining for embryo development. If not, decreasing hormone levels trigger menstruation.
Step-by-step explanation:
The Menstrual Cycle and Fertilization
During the menstrual cycle, if an egg (oocyte) is fertilized, it travels through the fallopian tube into the uterus, where it implants itself into the endometrium (the uterine lining). The implanted egg then begins the process of developing into an embryo. To support this development, the corpus luteum, formed from the ruptured follicle, secretes progesterone, a hormone crucial for maintaining the endometrium. This halts the menstrual cycle for the duration of the pregnancy. Conversely, if the egg is not fertilized, the corpus luteum degenerates, leading to a drop in progesterone levels. This hormonal decrease triggers the endometrium to break down, resulting in the shedding of the uterine lining through menstruation, and the menstrual cycle begins anew.
Estrogen levels impact the menstrual cycle too. After menstruation, estrogen levels rise, leading to the proliferative phase where the endometrium begins to rebuild itself. However, without a fertilized egg, estrogen and progesterone levels eventually fall, and the cycle starts over. The menstrual cycle is a complex interplay of hormones, with estrogen and progesterone being key players in preparing for and supporting a potential pregnancy, or in triggering menstruation when fertilization does not occur.