One of the largest-ever studies of menstrual cycle characteristics shows that common assumptions about when ovulation occurs may be at variance with what happens to women in the real world. Published in Digital Medicine, the findings are based on data on more than 600,000 ovulatory cycles accumulated from 124,646 users of a mobile app.
For the study, researchers from Sweden and UK analyzed information from the database of Natural Cycles, a fertility awareness-based contraceptive method designed to help couples use a woman’s dates of menstruation and basal body temperature to determine dates on which she is most fertile. It has more than 1 million users worldwide and can be used to prevent or plan a pregnancy.
Of the 1.4 million cycles recorded by eligible app users in Sweden, the UK and the United States, 612,613 were chosen for the analysis after ruling out cycles that fell outside the 10-to 90-day accepted cycle length, were anovulatory, or resulted in pregnancy. Users ranged in age from 18 to 45 with a mean age of 30.3 and a mean body mass index (BMI) of 23.6.
The authors compared the cycles recorded by the app with two reference databases: a sample of 688 cycles and a sample of 327 cycles. The goal of the study was to investigate associations between menstrual cycle characteristics and cycles length, age and BMI. The app collected information on those parameters as well as basal body temperature and luteinizing hormone (LH) levels.
Mean menstrual cycle length was 29.3 days, mean follicular phase length was 16.9 days, and mean luteal phase length was 12.4 days. For each year from age 25 to 45, 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 (CI 0.19-0.20, R2 = 0.99.
Mean variation in cycle length per woman was 0.4 days or 14% higher in women with a BMI > 35 compared with women with a BMI of 18.5 to 25. In women with BMI of 15 to 18.5, mean bleed length was longer by 0.2 days or 5% (95% CI 0.18-0.22 days) whereas women with BMI > 35 had more cycle length variation by 0.4 days or 14% (95% CI 0.3-0.5 days) and longer follicular phase length by 0.9 days or 5% (95% CI 0.8-1.0 days).
The authors said that there is “a common belief” that ovulation occurs on day 14 but their analysis indicates that in the “real world” for most women, that may not be the case. They also noted that while many clinicians believe the luteal phase is 14 days long, the mean in their study was 12.4 ± 2.4 days with a range of 8.0 days in cycles of 15 to 20 days and 12.9 days in cycles of 36 to 50 days.
The study’s results, the investigators said, “only scratch the surface of what can be achieved” with fertility apps, given the more than 100 that are available to download for free, while acknowledging that because the population was derived from users of a single app, the findings may not be representative of the wider population.