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You’re used to your period arriving like clockwork, but it suddenly came early. It’s annoying and a bit concerning. What gives?

A woman’s menstrual cycle usually runs anywhere from 21 to 39 days, while Dot visits for 2 to 7 days. Scientifically, the “menstrual cycle” is counted from the first day your period appears to the first day it appears the following month.

There’s no need to worry if Mother Nature occasionally brings your flow on a bit early. Our bodies usually sort things out on their own. But if your periods frequently show up less than 21 days apart, check with your doctor to make sure everything’s OK.

Here are 15 reasons why your period might be showing up early.

Let’s just get this one out of the way first. One to 2 weeks after a sperm meets up with your egg, the fertilized egg snuggles into your uterine lining and implants.

During implantation, you might see some red dots on your panties or experience cramping and wonder, “Am I pregnant, or is my period early?”

This life-altering question can be answered easy-peasy. Grab a couple of drugstore pregnancy tests. If one reads negative, wait a few days and then try the other one. You might have stalked the stork too early.

Ultimately, your doctor can help determine your status and whether there’s anything in your lady parts that needs tending.

An early period could signify a miscarriage — nature’s way of stopping a pregnancy that isn’t developing correctly.

Miscarriages are heartbreakingly common. About 10 to 15 percent of known pregnancies end in miscarriage. Often a miscarriage happens before you even realize you’re pregnant.

If the pregnancy is further along, you might have passed pink discharge, blood clots, or bits of tissue. If so, it’s best to see your doctor right away. They will likely do a pelvic exam and an ultrasound to find out what’s going on.

If you’ve had sex without a condom or another barrier and used emergency contraception (EC) to keep the babies at bay, you could be experiencing an early period as a result.

EC comes in two forms: a pill or a copper intrauterine device (IUD) inserted in your uterus. EC works by preventing or delaying ovulation.

An EC pill can trigger some mild side effects that shouldn’t last long. Most women get their period within a week of its expected arrival, so that could make the Red Moon rise early.

Common side effects include:

  • headaches
  • breast pain
  • abdominal pain or nausea
  • tiredness
  • dizziness

The copper IUD rests in your uterus. You might experience spotting or irregular periods after it’s inserted, but your body will likely adapt after a few months.

Copper IUDs can also trigger:

  • heavy periods
  • severe menstrual cramps
  • backaches

FYI

Regular use of Plan B can mess up your cycle. Standard birth control methods or condoms are easier on you and your system.

Bloody Mary’s early arrival could be related to stress. Prolonged pressure at work or deeply emotional events like weddings and funerals can throw your hormones out of whack.

Try chilling out by taking a walk, going to a yoga class, or spending time with friends.

The human body is sensitive! Gaining and losing weight can confuse your cycle. A significant change in weight might spur an early period.

Extreme dieting, an eating disorder, or gastric bypass surgery could send your body into starvation mode. This is when your body fuels only the most basic human needs, like breathing.

When that happens, producing reproductive hormones isn’t high on the to-do list, so your monthly cycle could ebb and flow.

Between ages 8 and 14, puberty usually kicks in. That’s when the reproductive hormones start flowing and jump-start the menstrual cycle.

If you’re the parent of a tween or teen who has recently had their first period, let them know it’s common for Aunt Flo to make a first visit and then stop showing up for a while.

As your child’s hormones fluctuate during this phase, the number of days between their periods may lengthen or shorten until their body finds a happy medium.

Puberty is when everything starts sprouting:

  • breasts
  • hair under the arms and at the groin
  • pimples
  • moods swings

Buckle up: Puberty can be a bumpy ride for both teens and their parents, but things will settle down in time. You’ve got this!

Early bleeding can also signal perimenopause, the transition phase leading to menopause. Remember all the fun signs that showed up during puberty? This is when they start to fade.

Perimenopause generally begins when a woman reaches her mid-to-late 40s. The shutdown of the baby-making factory takes about 4 years (or longer) and comes to a complete stop known as menopause.

During perimenopause, it’s normal for the hormones progesterone and estrogen to seesaw and cause irregular menstrual cycles. When this happens, ovulation may stop altogether, along with your monthly flow.

Perimenopause brings its own party:

  • heavier/lighter or irregular periods
  • vaginal dryness
  • hot flashes
  • irritability
  • difficulty sleeping

An intense exercise regimen can cause an early, late, or absent period. It’s usually the result of burning more calories than you take in.

This is because our bodies require a baseline of energy to produce reproductive hormones that trigger ovulation.

If this sounds like you, check in with your doctor.

There are many kinds of birth control on the market. Once you start taking a birth control pill, it runs the ovulation/menstruation show. The timing of your next “code red” depends on the first day you took the pill.

Other hormonal birth control options like IUDs and the Depo-Provera shot can shake up your flow for several months, triggering irregular periods and spotting.

Hormonal birth control may briefly cause:

  • cramping
  • sore breasts
  • headaches
  • nausea

Changes in your routine can make your hormones swing from the chandelier and your periods go wild.

People who frequently switch between working day shifts and night shifts (such as nurses) often experience irregular periods. If you’re a frequent traveler, changing time zones may have similar effects.

Researchers aren’t sure why, but it may be related to the continual disruption of your circadian rhythm and melatonin production. More research is needed to explore the possible connection between melatonin and reproductive hormones.

To help cope with alternating work schedules, try to get plenty of sleep in a dark, quiet room.

Women with diabetes often have more complicated menstrual cycles, since blood sugar levels tend to rise before menstruation.

Diabetes can also cause:

  • sudden weight loss
  • increased thirst
  • increased need to urinate, especially at night
  • slow healing

Endometriosis affects about 11 percent of American women ages 15 to 44. This ongoing condition occurs when some of the tissue that usually lines your uterus grows outside your uterus.

Endometriosis has been known to shorten women’s menstrual cycles, often causing early periods.

Side effects of this condition can include:

  • severe cramps
  • pain during or after sex
  • chronic lower back pain

A treatment plan developed with your doctor may help ease the condition’s side effects.

According to the American Thyroid Association, 1 in 8 women will develop a thyroid condition in their lifetime. The thyroid manages basic bodily functions like metabolism and your menstrual cycle.

When your thyroid produces either too much or too little hormone, it alters your periods, making them lighter, heavier, or irregular. Thyroid disease can also stop your periods for several months or longer, a condition called amenorrhea.

In addition to early menstruation, you may also experience:

  • lighter or heavier periods
  • a faster or slower heart rate
  • weight loss or gain
  • difficulty sleeping

Check with your healthcare provider if you’ve missed three periods in a row.

Sexually transmitted infections (STIs) like chlamydia and gonorrhea can cause spotting between periods or after sex. You may be unaware you have these bacterial infections. But if left untreated, they can cause pelvic inflammatory disease.

You need to see a doctor and take antibiotics to get rid of chlamydia or gonorrhea.

If you’re sexually active, it’s a good idea to get checked for STIs annually. The best way to avoid STIs is to use a latex condom every time you have intercourse.

STIs can cause:

  • burning
  • pain when peeing
  • pain during sex
  • stomachaches

If you have irregular periods, extra hair growing on your face or body, or acne or you’ve gained weight recently, ask your doctor about polycystic ovary syndrome (PCOS).

This condition affects about 1 in 10 women, and it indicates a hormonal imbalance. Unless you’re having difficulties getting pregnant, you might not know you have it.

It’s usually diagnosed through a medical exam and lab tests. Treatment for PCOS is usually a combination of hormonal therapies and lifestyle changes like weight loss.

If your period likes to arrive early, mark the start date in your planner for a few months.

There’s an app for that

You can also download apps to your phone, such as Clue, Eve, Glow, and Ovia. These trackers make it easy to log your daily symptoms and spot a pattern over time. The information gathered on the app is also useful to share with your doctor.

Make like a boy scout and be prepared!

Keep a stash of panty liners, pads, or tampons at work and in your purse so you’re always prepared. Your stash might also include period underwear that’s easy to wash. Grab some here.

Sweet dreams

Get 8 hours of sleep a night, since a wonky sleep schedule can piss off your cycle and trigger early periods. Getting enough sleep can be more challenging for folks who work night shifts, but it’s worth the effort.

Nosh to nourish

Eating well nourishes your brain and reproductive system. As we discussed above, those calories are necessary to help your body produce hormones.

If you find your weight creeping up, be aware that extra pounds can also interfere with your reproductive hormones. Check in with your doctor to see if losing weight could help get your cycle back on track.

Your doctor or a dietitian can help you put together an eating plan to meet your goals. You could also consider trying a new workout class with a friend or spending more time doing physical activities you enjoy.

Take self-care breaks

Stress can affect many aspects of your body, including your menstrual cycle. When things are getting tough at home or work, try doing breathing exercises, dabbing lavender oil on your pulse points, taking a walk, or watching your favorite show.

Sweat

Exercise is a great way to release some stress, but don’t go overboard. You need enough energy to fire up those critical gal hormones.

A few early periods are probably nothing to worry about. But if you’re feeling uncomfortable or you’re in pain, go see your doctor.

If you think you’ve had a miscarriage or aren’t quite sure if you’ve had one, check with your doctor.

If your periods continue to start every 21 days or less, take your notes and period app tracker data to your doctor and get checked out.

Bottom line

Our internal clocks aren’t always consistent, so if your period starts early, it’s usually OK.