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Why Period Might Come One Week Early

By Nancy Chen ; Updated July 27, 2017

A woman's menstrual cycle can be as unique as the woman herself. For some women, their period arrives on time every 21 or 24 days. However, a woman's menstrual cycle, which begins on the first day of one period and lasts until the first day of the next can span anywhere from 21 to 35 days. And a period can last anywhere from 2 to 7 days.

Menstrual cycles often develop consistency in a woman's 20s and will remain consistent until perimenopause. However, occasional irregularity in length or timing isn't uncommon nor it is necessarily the sign of a serious health issue.

What is a period?

Menstruation, or your period, occurs as a result of a woman's body preparing for pregnancy. During the menstrual cycle, an egg is released (a process called ovulation) from one of the ovaries. Meanwhile, hormonal shifts in the body prepare the uterus for pregnancy in the event the egg is fertilized by sperm to achieve conception.

If the egg isn't fertilized, the lining of the uterus is shed and exits through the vagina. The shedding and resulting bleeding is what's commonly called a “period.”

The American College of Obstetricians and Gynecologists defines irregular cycles as those that last less than 21 days or more than 45 days.And those irregularities may be caused by several different reasons.

Regular Cycles

The U.S. Department of Health and Human Services (HHS) defines the menstrual cycle as menstruations that come on a regular basis. On average, cycles occur every 28 days. In some healthy women, periods can occur every 21 to 35 days. In teenage girls, menstrual cycles vary even more; a “normal” cycle for girls can be from 21 to 45 days.

Exceptions to this “normal range” are when a girl first starts to menstruate and when a woman enters menopause (stops menstruating). Periods will be irregular due to expected hormone fluctuations.

Emergency Contraception

Your period may come earlier than usual if you use emergency contraception (EC) or the “morning after pill.”

That's because EC stops pregnancy by preventing the release of an egg. The hormones in emergency contraception also prevent fertilization of any eggs that have already been released.

Because the menstrual cycle is disrupted, your period may come earlier or later than usual. Periods should return to normal after one cycle following use of emergency contraception.

Birth control pills

Generally, "the pill" will cause your period to be regular. However, when first starting the pill, periods may arrive earlier than normal if your cycle lasted longer than 21 days.

Medical Conditions

Different medical conditions that may cause irregular periods include:

Eating disorders. Menstrual irregularities may result because of abnormal hormone levels. Sexually-transmitted diseases (STDs). Polycystic Ovary Syndrome (PCOS).The common hormonal disorder causes small cysts to form on the ovaries and can lead to irregular periods. Uterine fibroids. Noncancerous growths of the uterus that can cause heavy bleeding during your period as well as bleeding in between periods.


Extreme fluctuations in weight and excessive exercise can offset a woman's menstrual cycle by disrupting hormone production.

Aside from physical stress, emotional and mental stress can also affect hormonal levels and thus menstruation.

Regulating your cycle

Contraceptives are often prescribed to women who have irregular periods because they release specific and balanced levels of hormones the body needs to regulate periods.

Besides the use of contraceptives, getting the proper treatment for medical conditions, taking measures to minimize stress, maintaining a healthy weight and moderate daily exercise can help establish menstrual regularity.

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