• Early periods may be caused by lifestyle changes like stress, which alter your hormone production.
  • Early periods can also be caused by underlying conditions like PCOS and endometriosis.
  • Age-related hormonal changes, like puberty and perimenopause, may also cause early periods. 

Most people get worried if their period is late or they miss a period, but an early period can be just as concerning. There are multiple causes of an early period, ranging from stress to medical conditions like Polycystic Ovary Syndrome (PCOS).

Here are 11 reasons why your period might be early.

1. Stress

Stress may disrupt the timing of ovulation, which may cause a period to come early or late, says Aileen Gariepy, MD, MPH, an OB/GYN with Yale Medicine and associate professor at the Yale School of Medicine.

It's more common for stress to make a period late or skipped altogether, but it is also possible for stress to make a period come early, Gariepy says. 

2. Losing or gaining weight

If you've had drastic weight fluctuations, then this can alter hormone production, which can change the onset of menstruation, says Alla Vash-Margita, MD, OB/GYN at Yale Medicine and assistant professor at the Yale School of Medicine. This can cause your period to be early or late.

3. Strenuous exercise

Excessive, strenuous exercise can result in drastic weight fluctuations, which may alter hormone production, says Vash-Margita. This will affect ovulation and the timing of your period. 

4. Puberty

Puberty is the natural transition from childhood to adulthood. There is a change in hormones, including increased production of the female sex hormone called estradiol, which is stimulated by the hormones released by the brain, says Vash-Margita. 

As someone goes through puberty, their period may be irregular, which can result in early or late periods.

5. Perimenopause

Perimenopause typically occurs when a woman is in her 40s. Gariepy says in perimenopause, women have a diminishing supply of eggs left in the ovaries, and these eggs are poor quality. 

These eggs don't respond as well to hormone messages from the brain, which can cause infrequent ovulation or a lack of ovulation altogether. This causes shorter menstrual cycles and early periods or missed periods.

Additionally, Gariepy says that without progesterone, a hormone produced by ovulation, the uterus is more likely to shed in an unpredictable way, which can result in an early or late period.

6. Birth control

If you are using combined hormonal birth control that contains both estrogen and progestin — such as the pill, patch, or vaginal ring — the main reason your period would be early is if you missed a dose or several doses, Gariepy says. This is because the levels of the progestin you've been taking drops, triggering uterine bleeding.

If you are using a progestin-only birth control method such as the depo-medroxyprogesterone injection, arm implant, or IUD, Gariepy says early bleeding has to do with the thinning of the uterine lining. This is because the uterine lining doesn't build up.

If you are using a copper IUD without hormones, it's also common to experience side effects like unpredictable bleeding for the first 3 to 6 months as your body adjusts.

7. Emergency contraception

Gariepy says that emergency contraception pills such as Plan B work by delaying ovulation. This is because if no egg is released, then there is nothing for sperm to fertilize, preventing pregnancy. 

However, by disrupting ovulation, you may experience a change to your cycle which causes your period to be early or late. 

8. PCOS

Polycystic Ovary Syndrome (PCOS) is a hormonal disorder that may be associated with having excess male hormone (testosterone) levels, says Vash-Margita. This syndrome may cause ovulation to be irregular, which can result in early or late periods.

9. STIs

Some STIs — such as chlamydia, gonorrhea, and trichomonas — can cause irritation to the cervix and uterus, which may lead to bleeding, Gariepy says. 

Many people can mistake this bleeding for their periods. But with STIs, you'll likely also experience the following symptoms: 

  • Discharge 
  • Burning with urination
  • Pain with sex
  • Pelvic pain 

If you have these symptoms, it's important to see a doctor.

10. Endometriosis

Vash-Margita says endometriosis is when tissue normally located in the uterus grows outside the uterus. It's commonly associated with very painful periods and infertility. It may also result in bleeding between periods, which may be mistaken for an early period.

11. Implantation bleeding

Implantation bleeding occurs during the beginning of pregnancy when a fertilized egg attaches to the uterus, which triggers light bleeding or spotting. If you think you may be pregnant, be sure to take a pregnancy test. 

Insider's takeaway

There are many reasons that your period may be early, but it isn't always easy to determine the cause. If you're concerned about your early periods, see your OB-GYN so you can rule out certain conditions and get the answers you need.  

One thing to consider is that an early period might not be a period at all, but, for instance, bleeding in early pregnancy caused by the embryo implanting into the lining of the uterus, or because of changes in hormones and increased blood flow. 

If your period is early one month and you don't have any other symptoms, Gariepy says you shouldn't worry, especially if you're switching birth control methods or you are under a lot of stress. However, if you're experiencing other symptoms such as pain or unusual discharge, you should definitely see your OB-GYN.

If you are ever worried about something regarding your period, don't hold back from talking to your doctor. "We're always happy to talk about what's normal or potentially not normal and see if it's something that needs further investigating," Gariepy says. 

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