Final answer:
Menarche is the first menstrual period and a key event in female puberty, typically occurring at an average age of 12.5 years with considerable variation. It signifies the start of the reproductive years and onset of the menstrual cycle, which involves shedding the endometrium, known as menstruation.
Step-by-step explanation:
Menarche is a significant milestone in female puberty and refers to the first menstrual period. In the United States, the average age of menarche is approximately 12.5 years, although it varies due to factors like genetics and environment. The menstrual cycle, which begins at menarche, involves the shedding of the inner portion of the endometrium through the vagina, a process also referred to as menstruation or menses.
There is also a phase called the menses phase, which is the initial phase of the menstrual cycle. Menarche signifies the female reproductive years, which later conclude with menopause around the average age of 52. It is important to understand that a normal menstrual cycle can vary from 21 to 45 days in length, and the average menstrual period can last from two to seven days, showing that there are variations between and within individual women.