Real-world menstrual cycle characteristics of more than 600,000 menstrual cycles

Jonathan R Bull,Joyce Harper,Elina Berglund Scherwitzl,Kristina Gemzell Danielsson,Raoul Scherwitzl,Simon P Rowland

doi:10.1038/s41746-019-0152-7

Abstract

The use of apps that record detailed menstrual cycle data presents a new opportunity to study the menstrual cycle. The aim of this study is to describe menstrual cycle characteristics observed from a large database of cycles collected through an app and investigate associations of menstrual cycle characteristics with cycle length, age and body mass index (BMI). Menstrual cycle parameters, including menstruation, basal body temperature (BBT) and luteinising hormone (LH) tests as well as age and BMI were collected anonymously from real-world users of the Natural Cycles app. We analysed 612,613 ovulatory cycles with a mean length of 29.3 days from 124,648 users. The mean follicular phase length was 16.9 days (95% CI: 10–30) and mean luteal phase length was 12.4 days (95% CI: 7–17). Mean cycle length decreased by 0.18 days (95% CI: 0.17–0.18, R2 = 0.99) and mean follicular phase length decreased by 0.19 days (95% CI: 0.19–0.20, R2 = 0.99) per year of age from 25 to 45 years. Mean variation of cycle length per woman was 0.4 days or 14% higher in women with a BMI of over 35 relative to women with a BMI of 18.5–25. This analysis details variations in menstrual cycle characteristics that are not widely known yet have significant implications for health and well-being. Clinically, women who wish to plan a pregnancy need to have intercourse on their fertile days. In order to identify the fertile period it is important to track physiological parameters such as basal body temperature and not just cycle length.

Highlights

The menstrual cycle begins and ends with menstruation and is divided by ovulation into the follicular and luteal phases
This large analysis of menstrual cycle parameters provides insight into the physiology of the menstrual cycle amongst the general population, which is not widely known. It demonstrates significant variability in cycle and follicular phase length amongst a large group of women with wide age and body mass index (BMI) ranges. Using this large data set, our analysis reveals important information on menstrual cycle characteristics in a real-world population of women
Knowledge and understanding of the menstrual cycle, ovulation day and the fertile period is important for both individual women and healthcare professionals providing services in reproductive health

Summary

Introduction

The menstrual cycle begins and ends with menstruation and is divided by ovulation into the follicular and luteal phases. The luteal phase length was between 7 and 19 days in a sample of 28 day cycles.[1] With increasing age, cycle length reduces and the timing of ovulation becomes earlier; the variation of a woman’s cycle length reduces with age until menopause.[5,6,8,9]

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