Final answer:
Estrogen levels increase during the follicular phase of the menstrual cycle, as the growing follicles produce more estrogen to prepare the endometrium for potential pregnancy.
Step-by-step explanation:
During the follicular phase of the menstrual cycle, the level of estrogen increases. This phase starts on the first day of the menstrual cycle and lasts until ovulation, which typically occurs around day 14. It is characterized by the development of follicles in the ovaries, which start to release more estrogen. This hormonal increase is crucial in preparing the endometrium (the lining of the uterus) for a potential pregnancy.
Estrogen levels rise because the growing follicles, which encase the eggs, begin to secrete more estrogen into the bloodstream. The rise in estrogen also stimulates a surge in luteinizing hormone (LH) and follicle-stimulating hormone (FSH) shortly before ovulation. This results in a peak in estrogen levels just before the ovulatory phase. Therefore, the statement that menstrual cycle estrogen levels increase during the follicular phase is accurate.