As you probably learned back in middle school, menstruation is the monthly shedding of your uterine lining. Though it can be uncomfortable and sometimes inconvenient, your period is your body’s way of telling you that your reproductive system is working properly.

Just as every woman is unique, every woman’s period has its own personality. Some periods are short, others are long. Some are heavy, others are light.

After a few years’ worth of monthly bleeding, most women start to get a feel for their period’s frequency, duration, and flow. When something out of the ordinary happens — such as spotting between periods or an exceptionally heavy flow — it’s natural to wonder what’s going on.

Not really. The average woman’s menstrual cycle is 28 days long, and the average period lasts for three to five days, but there can be huge menstrual cycle variations from woman to woman.

“Three days is normal for some women, seven days is normal for others,” says Franklin Loffer, MD, executive vice president and medical director of AAGL (formerly known as the American Association of Gynecologic Laparoscopists). Similarly, the normal period flow can be heavier in some women than in others.

Rather than worrying about the length or frequency of your period, you need to consider whether anything has changed.

“A woman should really be tracking her own menstrual cycle, because it provides huge numbers of clues about whether something’s not right,” says Frances Ginsburg, MD, director of reproductive endocrinology at Stamford Hospital in Stamford, Conn., and assistant professor of clinical obstetrics/gynecology in the Columbia University College of Physicians and Surgeons.

Here are some common menstrual period changes, and what they might mean.

The big question if you’re not getting your period is — how old are you?

The cause of a missing menstrual period (called amenorrhea) varies by age. “To quit having periods at age 25 is a significantly different issue than quitting at age 50,” Loffer says.

For a woman in their 20s or 30s who is sexually active, pregnancy is always a possibility. “Even if a woman thinks she’s protected, that’s not an absolute guarantee,” Loffer says.

On the other hand, women in their 40s or 50s could be in perimenopause — the period surrounding menopause. As your ovaries slow their estrogen production, periods become less frequent. Periods also can get shorter or lighter during perimenopause. Once your periods stop for a full 12 months in a row, you’re in menopause. The average age for menopause is 51.

Another possible cause of missed periods is excessive exercise. Anywhere from 5% to 25% of female athletes work out so hard that they stop getting their periods. Called exercise-induced amenorrhea, this phenomenon is particularly common among ballet dancers and runners. Intense exercise affects the production and regulation of reproductive hormones involved in the menstrual cycle.

For similar reasons, women who have eating disorders such as anorexia nervosa can also stop getting their period. Severely restricting the amount of calories you eat suppresses the release of hormones your body needs for ovulation.

Other possible causes of missed periods include:

• Thyroid or pituitary gland disorders
• Disorder of the hypothalamus (brain area that assists with reproductive hormone regulation)
• Breastfeeding
• Obesity
• Oral contraceptives (although birth control pills will usually just make the periods lighter, rather than stopping them entirely)
• Stress
• Polycystic ovarian syndrome and other hormone imbalances
• Ovarian failure (the loss of normal ovarian function before age 40)
• Disease of the uterus (womb)

Most women only shed about 2 or 3 tablespoons of blood each month. Those with heavy periods (menorrhagia) can lose 5 or more tablespoons of blood monthly.

When you bleed excessively, you lose iron. Your body needs iron to produce hemoglobin, the molecule that helps your red blood cells transport oxygen throughout your body. Without enough iron, your red blood cell count will drop, leading to anemia. Signs of anemia include shortness of breath, unusually pale skin, and fatigue.

If you have a persistently heavy flow, see your doctor for a blood count to make sure you’re not iron deficient, Ginsburg advises. If so, you might need to take a supplement.

A number of conditions can increase your period flow, including:

You can gauge how heavy your period is by how many tampons or pads you’re using. Soaking through one or more sanitary pads or tampons every hour for a few hours in a row is a sign that you’re bleeding abnormally heavily.

Taking oral contraceptives can help regulate your menstrual cycle and reduce bleeding. If you use an IUD for contraception, your doctor may choose to insert a specific type of hormone-releasing IUD called Mirena to help reduce bleeding. Another option is a medicine called Lysteda, a pill that helps stop bleeding by increasing blood clotting.

If the bleeding continues, your ob/gyn might recommend that you have an ultrasound or other test to identify the source of the problem.

This is one period problem you shouldn’t ignore. “If you’re bleeding between periods, it should be investigated,” Loffer says.

Causes can range from something benign — such as having an irritated sore in the vaginal area or forgetting to take your birth control pill — to something as serious as an ectopic pregnancy or cancer. Visit your doctor for an exam.

Your period generally isn’t the most comfortable time of the month. Most women have cramps as the uterus contracts to shed its lining. Usually the discomfort is mild and it subsides in a day or two.

But for some women, the pain is so intense that they can’t get out of bed.

Painful periods are called dysmenorrhea. They can be accompanied by other symptoms, including diarrhea, nausea, vomiting, headache, or discomfort in the lower back.

Sometimes the pain is from the period itself, but it also can be caused by conditions like endometriosis and fibroids. To find the source of the problem, your doctor can do a pelvic exam and Pap test, as well as other diagnostic tests such as an ultrasound or laparoscopy.

Nonsteroidal anti-inflammatory drugs (NSAIDs) can help because they not only relieve pain, they also prevent the body from making prostaglandins — the chemicals that stimulate your uterus to contract during your period. Your doctor might recommend that you go on the pill or get an IUD, which can also reduce period pain. Fibroids and endometriosis are sometimes treated with surgery.

Any period issue that is out of the ordinary for you warrants a call to your doctor, especially if it makes you uncomfortable or keeps you from doing your normal activities. “If a woman feels that it’s interfering with her lifestyle, then she needs to address it,” Loffer says.

Definitely call your doctor if:

• Your periods used to be regular, but they’ve become irregular.
• Your period comes more often than every 21 days, or less often than every 35 days, for several cycles.
• You bleed for more than seven days straight.
• You stopped bleeding for 12 months in a row (menopause) and are now bleeding again.
• You are soaking through one or more pads or tampons each hour for several hours in a row.
• You have bleeding between periods.
• Your periods are very painful.