Menstruation is the approximately monthly shedding of the uterine lining; the blood and tissues leave the body. Menstruation is a normal, natural process that occurs in all healthy adult women who haven't reached menopause. Girls can begin to menstruate at any time between eight and eighteen; women might reach menopause anywhere between forty and sixty. Some women have their period every eighteen days, some every thirty-six; some women bleed for two or three days, some for seven or eight; all these and everything in between are normal.


One of the most common gynecological problems is an irregular menstrual cycle. But before you jump to the conclusion that you're irregular, it's important to remember that being regular doesn't mean your cycle is the same number of days each time. One month your cycle may be twenty-nine days and the next month it may be thirty-one days. This is still considered the normal. It's also normal to have a light flow one month and a heavy flow the next. Another common misperception about irregular cycles is that unless you have a period every four weeks (again, the statistical average) you're irregular. This is not true. Some women menstruate every three weeks, which is normal for them; some menstruate every five weeks, which is also normal for them. The only time you should be concerned is if your period consistently: for instance, three weeks, then four weeks, then five weeks, then three weeks. When this happens, it's usually a sign that you're not ovulating regularly. This is common in young girls after they first begin menstruating. If your period jumps around once or twice a year, there isn't anything to worry about. Occasional stress is usually the culprit when this happens.


Once in a while, women may skip a period and then experience a heavier flow with their next period. This is extremely common. Women who are trying to get pregnant, however, often fear that this is an early miscarriage – so mild it feels like a heavy period. This is usually not the case. Although it's possible for a pregnancy not to take and be expelled in the menstrual flow, it's rare and occurs in less than 1 percent of women. In most cases, skipping one period is caused by skipped ovulation, for reasons discussed below. The flow is heavier after a skipped period because the estrogen has been building up in the endometrium longer, and there is more lining than usual that needs to be shed. You would have built up two cycles' worth of lining, so the flow is naturally heavier than normal. That being said, you could have a lighter period or bleeding that fluctuates in duration or amount. It's not unusual to skip one or two periods a year; it is unusual to skip more than that, however.