Are your periods so heavy and long that they make your daily life hell, and you're wondering whether your blood loss is so heavy that it explains your anemia symptoms? Or have you recently started menstruating, and you would simply like to know how long periods usually last and how much blood most women lose during menstruation?
Normal Menstrual Flow: What To Expect
If you're wondering whether your menstrual cycle is normal, there are three basic aspects to examine:
- The duration of your periods
- The severity of your menstrual flow
- The duration of your menstrual cycle
How Long Periods Should Last
Most girls get their first period when they are 12 or 13, though everyone's different and you don't need to worry unless you still didn't get your period by age 16. When your first period arrives, it is most likely to last between two and seven days. [1, 2] As your menstrual cycle becomes more regular over time, the duration of your flow will also settle into a predictable pattern, and most women experience menstrual flow that lasts between three and five days. Some women will menstruate for only two days or up to seven days . It is very rare to menstruate longer than eight days .
It is interesting to note that the first three days of a period tend to feature the heaviest menstrual flow in women who menstruate for five days, and that periods tend to be shorter and lighter during anovulatory cycles. Since adolescent girls are more likely to experience cycles during which they do not ovulate, their periods may be lighter and shorter. 
How Heavy Should Your Menstrual Flow Be?
Menstrual fluid is more than blood alone — it also includes endometrial tissue and vaginal secretions. A woman will lose up to 60 ml of menstrual fluid over the course of one normal period. If you lose between 60 and 100 ml, your menstrual flow is classified as "moderately heavy", while anything over 100 ml is considered excessive. 
It is, however, pretty hard to accurately estimate the volume of your menstrual fluids unless you choose to use a reusable menstrual cup as your sanitary method. These cups contain measurements and those women who are interested in finding out how much fluid they really lose during a period can make a note of the volume present in the cup every time they empty it.
How Long Should Your Menstrual Cycle Last?
Cycles are counted from the first day of menstrual flow to the last day before your next period, and normal cycles can least anywhere between 21 and 35 days . It is quite normal to experience some variation in the length of your menstrual cycle, but women with extremely irregular cycles are advised to seek medical advice.
I Think My Menstrual Bleeding Is Abnormal: What Now?
If your periods are so heavy, or so prolonged, that they cause you discomfort severe enough to interfere with your daily functioning, asking your OBGYN to take a look at you is always a good thing — severe bleeding can cause anemia and related unpleasant symptoms, including chronic fatigue.
Unusually heavy and prolonged menstrual bleeding is called menorrhagia, and it can be caused by:
- Pelvic diseases like uterine fibroids or adenomyosis (where endometrial tissues embed themselves into the uterine muscle).
- Systemic medical conditions, such as hypothyroidism or blood disorders.
- Dysfunctional uterine bleeding, something that can be diagnosed if the other two causes are ruled out. 
In the latter case, the Mirena intrauterine system, a birth control method that releases hormones and helps bring your excessive menstrual bleeding under control, is one possible treatment . Endometrial ablation, a procedure during which the lining of your uterus is removed, can also treat dysfunctional uterine bleeding. It is, however, not suitable for women who would still like to become pregnant in the future. 
More conservative treatment can combine medication that aims to reduce bleeding  with over the counter or prescription painkillers to help manage the pain that many women with menorrhagia experience.
The Bottom Line
If the symptoms of abnormal periods — bleeding for more than a week or a flow heavier than 100 ml — sound familiar to you, know that you do not have to suffer periods from hell for the rest of your reproductive life. The first step to more bearable menstrual periods is getting a diagnosis, though, so see a doctor as soon as you can.