Periods, also known as menstruation, occur when your uterus sheds its lining, causing blood and mucus to discharge from the vagina. 1 This usually occurs once a month if you are not pregnant and. Understand how to track your menstrual cycle and what to do about irregularities. Do you know when your last menstrual period began or how long it lasted? If not, it might be time to start paying attention. When you track your menstrual cycles, you learn what's typical for you. What is a normal menstrual cycle? Your period — and the PMS symptoms that can accompany it — is just one part of your monthly menstrual cycle. Menstruation — aka having your period — is when blood and tissue from your uterus comes out of your vagina. It usually happens once every month. Learn what periods are, when they start, and how they happen. Also, get tips about cramps, PMS, and period products. Menstruation is the most visible phase of the menstrual cycle and its beginning is used as the marker between cycles. The first day of menstrual bleeding is the date used for the last menstrual period (LMP). period, epoch, era, age mean a division of time. period may designate an extent of time of any length. epoch applies to a period begun or set off by some significant or striking quality, change, or series of events. era suggests a period of history marked by a new or distinct order of things. Menstruation (or period) happens monthly. As the uterus sheds its lining, vaginal bleeding follows. Menstrual blood is made of blood and tissue. PERIOD. is a global youth fueled non-profit which strives to eradicate period poverty and stigma through advocacy, education, and service. PERIOD definition: 1. a length of time: 2. a fixed time during the life of a person or in history: 3. in school, a…. Learn more.