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
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 called ovulation). 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
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
A Normal Period Doesn't Have to Be Regular
Some girls' periods arrive like clockwork. Others get theirs
at slightly different times each month. Many girls get regular
periods most of the time, but occasionally skip a period or get an
extra period during times of pressure or stress. In fact, you may
notice that when you go on a trip or have a major change in your
schedule your period is late. All of this is perfectly normal.
It's also normal for the number of days a girl has her
period to vary. Sometimes a girl may bleed for 2 days, sometimes it
may last a week. That's because the level of hormones the body
manufactures can be different from one cycle to the next, and this
affects the amount and length of bleeding.
So how can you tell when you're about to get your period? If
your cycle is not regular, you'll want to pay attention to the
clues your body may give you. These include:
- back cramps or stiffness
- heavier breasts or breast soreness
- acne breakouts
- disturbed sleep patterns
- mood swings
Most of the time, irregular periods are part of the normal
changes that can happen when you're a teen. At some point as
you grow, your cycle will probably settle into a recognizable
pattern. This usually happens by 3 years after your first
However, some teens may develop irregular periods - or stop
having periods altogether - as a result of certain medications,
excessive exercise, very low body weight, or not eating enough
calories. Others may develop problems as a result of a hormone
imbalance. For example, disorders of the thyroid gland can cause
menstrual irregularities if the levels of thyroid hormone in the
blood become too low or too high.
Some women have irregular periods because their bodies produce
too much androgen, which is a hormone that causes increased muscle
mass, facial hair, and deepening of the voice in males and the
development of pubic hair and increased height in girls. High
amounts of androgen can also cause hair growth on the face, chin,
chest, and abdomen, and is sometimes associated with excessive
If you have any of these problems, or if your periods are
irregular for 3 years or more, see a doctor. The doctor may
prescribe hormone pills or other medications or recommend lifestyle
changes that can help you to have regular periods.
It's important to see a doctor if you're sexually active
and have missed a period. This could be a sign of pregnancy. You
should also see your doctor if you start having periods that last
longer than 7 days, are heavy, or are accompanied by severe
cramping or abdominal pain.
In the meantime, if your periods are irregular, try keeping some
pads or tampons in your backpack, just so you'll have them
handy in case your period comes when you're not expecting
Larissa Hirsch, MD
Date reviewed: April 2007
Note: All information is for educational purposes only. For specific medical advice,
diagnoses, and treatment, consult your doctor.
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