Final answer:
Menarche is the term for a girl's first menstruation, marking a significant milestone in puberty when the female body becomes capable of reproduction. It typically occurs around 12.4 to 12.5 years of age, with considerable variation due to genetic and environmental factors.
Step-by-step explanation:
Menarche: The First Menstrual Period
The first menstruation in a pubertal female is known as menarche. Menarche signifies the onset of menstruation, which is the shedding of the inner portion of the endometrium out through the vagina. This process is a significant event in puberty and marks a girl's transition into reproductive maturity. In the United States, the average age of menarche is about 12.4 to 12.5 years, though ages between eight and 16 are considered normal due to variations influenced by genetic and environmental factors such as diet.
Menarche is a natural part of the female reproductive cycle, starting as the ovaries begin to release eggs cyclically, leading to the shedding of the endometrium if fertilization does not occur. This menstrual phase, colloquially referred to as a 'period,' is part of a larger menstrual cycle that prepares the female body for potential pregnancy.