Even though girls get their periods on a cycle, that cycle can take different amounts of time each month. For example, a girl might get her period after 24 days one month and after 42 days the next. These are called irregular periods.
Irregular periods are very common, especially in a girl's first few years of getting her period.
What Are Regular Periods?
Most girls get their first period between the ages of 10 and 15, but some get it earlier and some later. The first period is known as menarche (pronounced: MEN-ar-kee).
A girl's monthly cycle is the number of days from the start of her period to the start of the next time she gets her period. You often hear this is a 28-day cycle. But 28 is just an average figure that doctors use. Cycle lengths vary — some are 24 days, some are 34 days. 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.
Early in a girl's cycle, her ovaries start preparing one egg. 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.
About 2 weeks before a girl gets her period, the egg is released from the ovary (this is called ovulation). The egg travels through the fallopian tube into the uterus. If the egg isn't fertilized by sperm, it starts to fall apart. Then 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." The first day a girl's period comes is Day 1 of her cycle.
A girl's body may not follow an exact schedule. It's common, especially in the first 2 years after a girl starts getting her period, to skip periods or to have irregular periods. Illness, rapid weight change, or stress can also make things more unpredictable. That's because the part of the brain that regulates periods is influenced by events like these. Going on a trip or having a major change in schedule can also make your period come at a different time than expected. 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 makes can be different from one cycle to the next, and this affects the amount and length of bleeding.
If My Period Is Irregular, How Do I Know When I Will Get It?
If your cycle is not regular, you'll want to pay attention to the clues your body may give you that your period is coming soon. These may include:
Keep some pads or tampons in your backpack or purse, just so you'll have them handy in case your period comes when you're not expecting it. You may even want to carry an extra pair of underwear.
What Causes Irregular Periods?
Most of the time, irregular periods are part of the normal changes that can happen when you're a teen. As you get older, your cycle will probably settle into a recognizable pattern.
Sometimes, irregular periods can be caused by some medicines, exercising too much, having a very low or high body weight, or not eating enough calories.
Hormone imbalances can also cause irregular periods. For example, thyroid hormone levels that are too low or too high can cause problems with periods. Some girls have extra androgen, a hormone that can cause hair growth on the face, chin, chest, and abdomen. Extra androgen can also makes girls gain weight and have irregular periods.
Girls who are pregnant also will not get their periods.
Should I Worry About Irregular Periods?
Talk to your doctor if you have had sex and have missed a period because you could be pregnant. Also let the doctor know if:
You were having regular periods that then become irregular.
You stop getting your period.
You have extra hair growth on the face, chin, chest, or abdomen.
You start having periods that last longer than 7 days, are heavy, or are coming more often than every 21 days.
Your period comes less often than every 45 days.
You have severe cramping or abdominal pain.
You have bleeding in between your periods.
Your periods are irregular for 3 years or more.
The doctor may prescribe hormone pills or other medicines, or recommend lifestyle changes that can help you to have regular periods.