Most menstrual cycles aren’t 28 days — and that’s normal

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Many believe the average menstrual cycle is 28 days, but new research has found only a small percentage of women fall within that range.

Research out of Sweden and the U.K. recently published in the journal npj Digital Medicine found that only 13 per cent of cycles are 28 days in length. Instead, the study concluded the average cycle is 29.3 days long.

Using a menstrual tracking app, researchers studied more than 600,000 ovulatory cycles from 124,648 users based in the U.K., U.S. and Sweden. They also found that despite the common belief that ovulation usually occurs on day 14 of a cycle, this is not the reality for most women.

READ MORE: All about your period — what’s normal, and when you should see a doctor

The follicular phase — which is when follicles in the ovary mature, resulting in ovulation — can range from 12 days to 19 days, researchers wrote.

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This variation in cycle length is completely normal, says Dr. Yolanda Kirkham, an OBGYN at St. Joseph’s Health Centre Toronto and Women’s College Hospital.

“Not everybody is 28 days,” Kirkham said. “Twenty-eight is an average, but [cycles] can range anywhere from 21 to 35 days.”

Why menstrual cycles vary

Your cycle length depends on when you ovulate, Kirkham said. That’s what triggers your cycle.

“What happens is the ovary releases the egg… and then two weeks later, if there’s no pregnancy, that’s when people bleed,” Kirkham said.

Click to play video: 'Period myths debunked'
Period myths debunked

That means if someone ovulates on day 14, their period will likely come on day 28, she said. But if a person ovulates earlier that month on day 10, their period would come sooner.

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What’s more, cycles can vary from month to month. This means even if you had a 30-day cycle one month, you could have a 28-day cycle the next, Kirkham said.
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Aside from the physiological trigger of ovulation, there are other factors that affect menstrual cycle length. This can include things like stress, nutrition and changes in weight, Kirkham said.

Hormonal conditions, like polycystic ovarian syndrome, can affect one’s cycle, too. So can chronic diseases.

READ MORE: Endometriosis affects 1 in 10 women – here’s how to recognize the symptoms

“If someone is quite ill, it’s not going to be their bodies’ priority to have pregnancy, so their periods can disappear,” Kirkham said.

“And then there’s also structural things, like if people grow polyps, fibroids, or they have infections or cancer.”

Age is another factor. When a person first gets their period, it can take two to three years for their cycles to regulate, Kirkham said.

“It’s very normal for young women to sometimes have skipped cycles or bleed excessively as well, because the ovulation process is not in order yet,” she said.

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Once people hit reproductive age, cycles are more regular, Kirkham said. This means cycles can be anywhere from 21 to 35 days.

“In the perimenopause time — so the late 40s, early 50s — it’s kind of like puberty and again we’re losing some of that ovulation,” she said. “You lose the proper timing of the periods.”

If people are on the birth control pill, their periods become much more regulated and predictable. This is because the pill causes “withdrawal bleeds,” Kirkham said, which are caused by a drop in hormones.

READ MORE: ‘My mood plummets’ — When PMS symptoms could be something more

“Withdrawal bleeds doesn’t mean they’re not healthy or safe,” she added. “I call them ‘period control pills’ because they are actually very effective for reducing flow and reducing pain — especially in light of abnormal periods.”

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What’s abnormal?

Kirkham says people should see their doctors if they experience excessive bleeding, go more than three months without a period and are not pregnant, or have severe period-related pain.

“A little bit of mild cramps —  something some over-the-counter painkillers can help with — is normal, but anything where you have to continuously take painkillers or you’re missing school or work is abnormal,” she said.

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People should also seek medical advice if they are concerned about fibroids or polyps.

“These types of things can be checked out by your doctor with an ultrasound,” Kirkham said. “They may not necessarily need to do an invasive examination.”

Kirkham also recommends women read more about menstrual cycles on the Society of Obstetricians and Gynaecologists of Canada’s website.

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