A female’s first menstrual period, known as menarche, occurs during puberty. Menstruation indicates that the body is mature enough for reproduction. About 98% of girls begin their periods by age 15.

It's not an official medical diagnosis, but rather a form of premenstrual syndrome (PMS). More than 90 percent of people who menstruate say they get symptoms like bloating, moodiness, and headaches a ...