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New European research has found that only 13 percent of women have menstrual cycles lasting 28 days, which researchers say could help couples better understand when pregnancy is more likely to occur.

Led by University College London (UCL) and Natural Cycles, a contraceptive app, the new study is one of the biggest to date to look at menstrual cycles, analyzing data gathered from the app on over 600,000 menstrual cycles and 124,648 women aged 18 to 45 from Sweden, the U.S. and the U.K.

The researchers studied whether the women’s menstrual cycle characteristics were associated with age, body mass index (BMI) and body temperatures.

The women all had a BMI between 15 and 50, and none had used hormonal contraception in the 12 months before they registered on the app. Women who reported menopausal symptoms or a pre-existing medical condition such as polycystic ovarian syndrome, hypothyroidism or endometriosis were excluded from the study.

The findings, published in npj Digital Medicine, showed that menstrual cycles are considerably varied, with the average cycle length 29.3 days. Only around 13 percent of cycles are 28 days in length, and 65 percent of women in the study had cycles that lasted between 25 and 30 days.

The team also found that average cycle length decreased between the ages of 25 and 45 as the women aged. The length of the cycle also varied more among obese women.

“Our study is unique in analyzing over half a million cycles and re-writing our understanding of the key stages. Traditionally studies have concentrated on women who have approximately 28-day cycles and these studies have formed our understanding of the menstrual cycle,” said co-author Professor Joyce Harper. “For the first time, our study shows that few women have the textbook 28-day cycle, with some experiencing very short or very long cycles. We studied all women who used the app.”

“We also demonstrate that ovulation does not occur consistently on day 14 and therefore it is important that women who wish to plan a pregnancy are having intercourse on their fertile days. In order to identify the fertile period, it is important to track other measures such as basal body temperature as cycle dates alone are not informative.”

Co-author Dr. Simon Rowland, Head of Medical Affairs at Natural Cycles, said, “Given the variations in cycle length and follicular phase length that we have described, especially for cycles outside the average range (25-30 days), an individualized approach to identify the fertile window should be adopted. Apps giving predictions of fertile days based solely on cycle dates could completely miss the fertile window and it is therefore unsurprising that several studies have shown that calendar apps are not accurate in identifying the fertile window.” NVG

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