BACKGROUND AND PURPOSE:

• Bull et al. (npj Digital Medicine, 2019), using data collected via an app, sought to determine menstrual cycle characteristics

METHODS:

• Prospective data analysis
• Data derived from users of the Natural Cycles app
  • Daily BBT (sublingual) | Cycle by cycle dates | Urinary LH
  • Age and BMI information also collected via the app (mandatory fields)
• Inclusion criteria for participants
  • Provided consent
  • 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

RESULTS:

• 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

CONCLUSION:

• 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.

Learn More – Primary Sources:

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

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