If a woman has a period that comes and goes, but the total amount of menstruation is within the normal range, it is usually physiologic. This is because the lining of the uterus flakes off intermittently during menstruation, not continuously. When the lining of the uterus flakes off, it is discharged from the vaginal opening. When menstrual blood is seen, but the lining has not flaked off, there may be no visible menstrual blood discharged from the vaginal opening, which can lead to the manifestation of menstruation being present at one time and absent at another. If a woman’s menstrual flow is particularly low and recurrent for more than 10 days, it is usually abnormal. It may be due to irregular vaginal bleeding caused by pregnancy, or it may be caused by atrophy of the corpus luteum, which requires medical attention.