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Many women don't have 28-day menstrual cycles, study finds

Women trying to get pregnant should understand their menstrual cycles and ovulation.
/ Source: TODAY

Many women were taught their menstrual cycles should last about 28 day. But a new study finds that isn’t accurate for most of them.

Only 13 percent of women have 28-day cycles, with 29.3 days being the average length, according to the paper, published in Nature Digital Medicine. This suggests that women’s fertility can vary from month to month.

“Our studies shows how individual women are, the range of menstrual cycle length and how this alters the day of ovulation,” Joyce Harper, head of the department of Reproductive Health and director of education at the Institute for Women’s Health, University College London, told TODAY via email.

menstrual cycle not actually being 28 days
A new study finds that most women don't have a 28-day cycle like they once thought. Getty Images

The authors looked at more than 600,000 ovulation cycles from more than 120,000 anonymous women, ages 18 to 45, using fertility apps from the United States, United Kingdom and Sweden. None of the women reported using hormonal contraception for a year prior to signing up for the app. Women with a diagnosis of polycystic ovarian syndrome (PCOS) or signs of menopause were excluded.

The study allowed the researchers to deeply investigate the women’s cycle patterns. Nearly 65 percent of them had cycles that lasted between 25 and 30 days. Women with obesity had longer cycles, though the participants had lower body mass indexes (BMI) than the average woman, which Harper said is a limitation to the study.

Understanding variability of women’s cycles can help them in many ways, whether they are trying to get pregnant or not.

“When a woman is trying to get pregnant, especially if it is taking longer than hoped, they need to know when THEY ovulate," Harper said. "It is so individual. Some ovulate earlier than day 14 and some later.”

Dr. Marie Menke, who was not involved with the study, said the paper reinforces what doctors have long observed: a 28-day cycle isn't the norm for many. Women's cycles often range anywhere from 21 to 35 days, but a lot of advice is based on the 28-day cycle.

"It is really just nice to see how much variety there is written out by a number," the division chief of reproductive endocrinology and infertility at UPMC Magee Women's Hospital in Pittsburgh told TODAY. "Women are very normal if they don’t fit that (28 day cycle)."

Dr. Christine Greves, who also was not a part of the study, agrees. What’s more, this understanding can help them better when trying to get pregnant. A longer cycle might mean that ovulation starts later, she said, not reliably on day 14, and this helps doctors better counsel their patients.

“There can be significant variability in cycle length and follicular phase and that the majority of women do not ovulate on day 14,” the OB-GYN at the Center for Obstetrics and Gynecology at Orlando Health told TODAY via email. “This article was nice because it helps others know that so they can take the needed measures to achieve a pregnancy if that is their desire.”

She hopes that women with varying cycles don’t get discouraged by their differences. Women with longer or shorter cycles still ovulate — just not on day 14.

“It helps women know they are not alone,” she said.

Having this knowledge should also help explain why some women who try ovulation apps may not get accurate results.

“I think they’re great for some,” Greves said. “If you are a perfect 28-day cycle and very regular, yeah it will really tell you when you’re ovulating.”

The experts agree that if women are trying to conceive, they can’t just rely on an app that solely uses a calendar.

“It is important for women to understand how their menstrual cycle works, whether they are trying to get pregnant or not,” Harper said.

“Using a fertility app that measures basal body temperature and/or luteinizing hormone (which increases about 40 hours before ovulation) and some record changes in the cervical mucus," she said, "These markers can pinpoint ovulation.”