We hear a lot about the menstrual "cycle," which can make it sound as though it happens like clockwork. And we say that a woman who gets her period every 4 weeks is "regular," as though there's something abnormal about women who don't. In fact, most women don't get their periods in exactly the same number of days after the last one.
The Menstrual Cycle
Most girls get their first period between the ages of 10 and 16. This is known as menarche (pronounced:meh-nar-kee).
Doctors often talk about a girl's monthly cycle — the number of days from the start of her period to the start of the next one — in terms of a 28-day cycle. But 28 is just an average figure that doctors use. Women's cycle lengths vary — some have a 24-day cycle, some have a 34-day cycle. And a girl may notice that her cycles are different lengths each month — especially for the few years after she first starts getting her period.
The first day a girl's period comes is Day 1 of her cycle. Around Day 5, her pituitary gland tells her ovaries to start preparing one of the eggs they contain for release. One egg will mature completely. At the same time, the lining of the uterus becomes thick to prepare a nesting place for a fertilized egg in the event that the girl becomes pregnant.
On or about Day 14 of a 28-day cycle, the egg breaks loose (this is calledovulation). The egg makes its way through the fallopian tube into the uterus. If the egg hasn't been fertilized by sperm, it starts to fall apart. About 2 weeks later, the lining and egg leave a girl's body as her period and the whole thing starts all over again — that's why we use the word "cycle."
All this sounds very neat and orderly. But a girl's body may not follow this schedule exactly. It's not unusual, especially in the first 2 years after menarche, to skip periods or to have an irregular menstrual cycle. Illness, rapid weight change, or stress can also make things more unpredictable because the part of the brain that regulates periods is influenced by events like these.