THURSDAY, Sept. 5, 2019 (HealthDay News) — Menstrual cycle length per woman is varied, and tracking physiological parameters such as basal body temperature is key to identifying fertility windows when planning pregnancy, according to a study published online Aug. 27 in npj Digital Medicine.

Jonathan R. Bull, Ph.D., of Natural Cycles Nordic AB in Stockholm, and colleagues studied 612,613 ovulatory cycles with a mean length of 29.3 days for 124,648 participants, collected anonymously from real-world users of the Natural Cycles phone app.

The researchers found that the mean follicular phase length and mean luteal phase length were 16.9 and 12.4 days, respectively. Among women aged 25 to 45 years, the mean cycle length and mean follicular phase length decreased by 0.18 and 0.19 days, respectively, per year of age. For women with a body mass index (BMI) >35 kg/m², the mean variation of cycle length per woman was 0.4 days or 14 percent higher compared with women with a BMI of 18.5 to 25 kg/m².

“These findings demonstrate that the widely held belief that ovulation occurs consistently on day 14 of the cycle is not correct,” the authors write. “Clinically, 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 physiological parameters such as basal body temperature and not just cycle length.”

Several authors disclosed financial ties to Natural Cycles Nordic AB, which funded the study.

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