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Elizabeth and Mr. Darcy. Leia and Han. Amy and Hope. There are some epic love-hate relationships, but few are as legendary as the one between a person and their period.

Most periods last 4 to 7 days, while cycles typically range from 21 to 35 days. But menstruating folks know their menses can be finicky.

It’s actually normal for your cycle to seem as unreliable as WiFi on occasion. Also normal: wanting to self-diagnose when you’re ready to surf the crimson wave and it’s a no-show.

A heavier or lighter flow than usual, bleeding or spotting between periods or after sex, and a period that lasts longer than 7 days (or is early or late) are all considered “normal” menstruation problems.

But the following health and lifestyle factors can actually affect your flow and might require profesh help.

A chronic disease can upend many things in your life, and that includes Aunt Flo’s regularly scheduled visits.

It’s rare, but poorly controlled diabetes can lead to an irregular menstrual cycle. This is because changes in your blood sugar levels affect your hormones.

Celiac can also disrupt your period. The autoimmune disease causes inflammation that damages the small intestine, which can prevent your body from absorbing important nutrients.

One study found that nearly 20 percent of women with celiac have skipped 3 or more periods in a row, while less than 3 percent of women without celiac had the same experience.

For many women, the number on the scale is constantly in flux. But unintentionally losing 5 percent of your weight over 6 to 12 months or weighing 10 percent less than what’s considered a “normal range” for your height affects bodily functions like your cycle.

When the body doesn’t get the nutrients it needs to fuel itself, malnutrition occurs. An unbalanced diet, anorexia, bulimia, and exercise bulimia can all lead to malnutrition and cause irregular menstruation.

Eating disorders and excessive exercise also affect estrogen production and, as a result, your period.

Checking your BMI is a quick way to assess your weight, but keep in mind that it doesn’t measure muscle and could possibly overestimate your body fat.

Having obesity can cause Mother Nature to take a little vacay.

Why? People with obesity may produce excess estrogen — one of the (many) hormones that regulates our reproductive systems. Which, if you haven’t realized at this point, is just as persnickety as Goldilocks.

If you think your weight may be affecting your cycle, talk to your doctor. They’ll be able to determine whether or not it’s a factor. And if it is, they can help you reach a healthier weight.

Polycystic ovary syndrome (PCOS) is a condition that affects the ovaries. It causes high levels of androgen (a male hormone) to be produced. And as we’ve already learned, too many or too few hormones can put your lady bits out of commission.

PCOS can result in ovarian cysts and an irregular or MIA period. Other symptoms include:

  • acne
  • dark patches of skin on the neck, groin, and under breasts
  • excessive hair growth on the face and body
  • headaches
  • heavier periods
  • male-pattern baldness
  • weight gain

Although PCOS is a leading cause of infertility, a study found that nearly 70 percent of women with the syndrome hadn’t been diagnosed. If you have PCOS, birth control and the diabetes drug metformin can help regulate your period and some of the condition’s symptoms.

It’d be easier to list the things stress doesn’t affect than the things it does. On the latter list is your hypothalamus, which is the part of your brain that regulates your menstrual cycle.

When you’re stressed, your period may be shorter, longer, or more painful than usual. It could even stop altogether. Stress can also cause sudden weight loss, which impacts your cycle.

Managing stress is easier said than done, but lifestyle changes like exercising regularly, eating well, and getting enough sleep can help keep “shark week” on its regular schedule and under control.

Menopause typically takes place between the ages of 45 and 55. It ends when a person hasn’t menstruated for 12 consecutive months.

Early menopause occurs when someone between the ages of 40 and 45 goes through menopause.

Those who experience menopause symptoms sooner than 40 go through what’s called premature menopause. To help reduce the stigma, many doctors refer to it as “premature ovarian failure” or “primary ovarian insufficiency.”

But a premature visit from the Menopause Banshee is relatively rare, affecting less than 1 percent of women.

Hyperthyroidism or hypothyroidism — an overactive or underactive thyroid — could also be the culprit.

See, your thyroid regulates your metabolism, which impacts your hormones. And since our bodily functions are sensitive AF, any hormone imbalance can send your system into a tailspin.

Aside from menstrual issues, symptoms of an overactive thyroid include heart palpitations, weight loss, heat intolerance, and an increase in appetite, sweating, or bowel movements.

Weight gain, lethargy, depression, facial puffiness, constipation, or a slower heart rate could indicate an underactive thyroid.

Autoimmune disorders like Graves’ and Hashimoto’s diseases are among the most common thyroid issues. Symptoms can usually be managed or alleviated with medicine, but thyroid conditions tend to be life-long.

A late start to the red wedding doesn’t automatically mean your eggo is preggo, but it’s a possibility if you’re sexually active. If the “festivities” don’t begin in a few days, try taking a pregnancy test.

See your GP or gynecologist if you’ve missed three or more periods in a row and aren’t pregnant, or if your cycle becomes unpredictable. They can diagnose your AWOL or unpunctual period, or refer you to a specialist.

Consider logging the Red Baron’s arrivals and departures and any other health changes you notice — it’ll make it simpler to give your doctor the DL. (The iPhone Health app comes standard with a cycle tracker, but there are tons of options for both iPhone and Android.)

If you experience any of the following symptoms, contact your doctor immediately:

  • bleeding that lasts longer than 7 days
  • bleeding if you’ve already entered menopause and haven’t menstruated in a year
  • fever
  • nausea and vomiting
  • severe pain
  • unusually heavy bleeding

Sorry Bill Shakespeare, but “why is my period late?” is *the* question.

It’s not uncommon for your period to ghost you like a Tinder date every now and then; a study found that 5 to 35 percent of women experience irregular menstrual cycles.

But if your absentee or unreliable menses is giving you pause, go ahead and give your doc a ring.