Age and BMI information also collected via the app (mandatory fields)
Inclusion criteria for participants
Age: 18 to 45 years
BMI: 15 to 50
Not using hormonal contraception within the 12 months
Inclusion criteria for cycles
≥6 complete cycles (regardless if ovulatory)
Cycle length: 10 to 90 days
No abnormal temperature recordings for ≥50% of cycle days
124,648 users | 612,613 ovulatory cycles | Mean BMI 23.6 (range 15 to 50)
Mean cycle length: 29.3 days
Only 13% of cycles were 28 days, with mean follicular phase length of 15.4 days and mean luteal length of 12.6 days
Mean follicular phase length: 16.9 days (95% CI, 10 to 30)
25–30 days: 15.2 days
31–35 days: 19.5 days
21–24 days: 12.4 days
Mean luteal phase length: 12.4 days (95% CI, 7 to 17) with a range of 8 days (15-20 days) to 12.9 days (36-50 days)
Mean cycle length
Decreased by 0.18 days per year of age from 25 to 45 years (95% CI, 0.17 to 0.18)
Mean follicular phase length
Decreased by 0.19 days per year of age from 25 to 45 years (95% CI, 0.19 to 0.20)
Mean variation of cycle length per woman
14% higher in women with a BMI >35 vs BMI 18.5 to 25
The average menstrual cycle is actually 29.3 days not the typical 28 days
Age is negatively correlated with cycle length
Ovulation does not occur consistently on day 14 | Luteal phase varies and is less than 14 days
Apps that base their fertile window on the traditional 28 day cycle may be incorrect
The authors state that based on the above data
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 individualised approach to identification of the fertile window should be adopted.
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