Final answer:
Menarche is the first menstrual period in girls, a significant event indicating the onset of reproductive capability during puberty. It generally occurs around the age of 12.5 years but can vary widely.
Step-by-step explanation:
Menarche is the term used to describe the first menstrual period in girls. It is a significant event during puberty, marking the onset of a girl's reproductive capabilities. In the United States, the average age for menarche is typically around 12.5 years, but it can occur anywhere from 8 to 16 years of age and still be considered normal. The timing of menarche can be influenced by a variety of factors, including genetics and environmental aspects such as diet. Menarche signifies that the ovaries have begun the cyclic release of eggs, paving the way for fertility.