Final answer:
The menstrual cycle is a complex series of hormonal changes preparing for fertilization. If fertilization occurs, the body halts the cycle to support the embryo. If not, hormonal drops initiate menstruation and a new cycle begins.
Step-by-step explanation:
Understanding Menstrual Cycle and Fertilization
During a woman's menstrual cycle, her body undergoes various hormonal changes to prepare for potential fertilization. If the egg is fertilized, it travels through the fallopian tube and implants into the thick uterine lining, halting the monthly cycle until the end of the pregnancy. This process is supported by the corpus luteum, which secretes progesterone to maintain the endometrium. However, if fertilization doesn't occur, the corpus luteum degenerates, leading to a decrease in progesterone and estrogen levels. Consequently, the endometrium breaks down and is shed, initiating the next menstrual cycle. The decrease in progesterone triggers the hypothalamus to send GnRH to the anterior pituitary, releasing FSH and LH, and hence restarting the cycles.
Additionally, if no implantation occurs, the egg eventually degrades and is possibly shed during menstruation. The ovarian cycle begins with the follicular phase, where FSH and LH contribute to follicle growth and egg preparation for ovulation. During this phase, follicles release estrogens and a small quantity of progesterone that support the endometrium. Should pregnancy not occur, the uterine lining is sloughed off, followed by a rise in estrogen levels, marking the proliferative phase and regeneration of the endometrium.