Final answer:
Menarche is the first menstrual period in a pubertal female, marking the start of her reproductive years, and it typically occurs around age 12.5. Irregularity in menstrual cycles can be common after menarche.
Step-by-step explanation:
The term menarche refers to the first menses or the first menstrual period in a pubertal female. Menarche marks a significant milestone in female puberty and signifies the beginning of a female's reproductive years. This event typically occurs around the age of 12.5 years in U.S. girls, but there is considerable variation due to genetic and environmental factors. Menstruation or menses is the phase of the menstrual cycle where the inner portion of the endometrium is shed through the vagina, and it is characterized by shedding of the built-up endometrial tissue if fertilization has not occurred. Menarche thus represents the start of monthly cycles where the endometrium builds up in preparation for a possible pregnancy and then sheds if fertilization does not happen.
It is normal for the menstrual cycle to be irregular during the first few years after menarche, as the body adjusts to the new hormonal rhythms. Menstruation can also be associated with cramping and pain, a condition known as dysmenorrhea. In the provided case study, Gabrielle's irregular menstrual cycle, which is common in young women shortly after menarche, may require medical evaluation to exclude other potential causes of irregularity. Lastly, the mention of a woman experiencing minor bleeding three weeks after her menstrual period could indicate a phenomenon known as 'mittelschmerz' or ovulation bleeding, which sometimes occurs during the fertile window of a woman's cycle.