Optimizing basal body temperature measurement for cycle diagnostics: a comparison of different methods in female...

Accurate detection of ovulation is essential for menstrual cycle-based training periodization for female athletes. Although body temperature tracking is a non-invasive method for this purpose, its reliability can vary depending on the measurement site and timing. This study aimed to compare the utility of different body temperature measurement methods and measurement times for ovulation detection.

Seventeen recreationally active women tracked one menstrual cycle (October 2023 to February 2024) using continuous intravaginal core body temperature measurement as the reference method. Additional temperature measurements (sublingual, rectal and external ear) were taken at two points in time: 6 a.m. and upon waking. Ovulation was additionally confirmed using luteinizing hormone urinary tests. The post-ovulatory temperature increase was analyzed using both the validated Vollman method and the practical "three over six" rule for the four different methods and two different time points. Agreement of the measurement methods and times was assessed via Bland-Altman plots and concordance correlation coefficients.

The continuous intravaginal temperature sensor consistently identified ovulation with acceptable accuracy, detecting all ovulations and showing the largest temperature rise from follicular to luteal phase (0.31 ± 0.18 °C). Sublingual 6 a.m. measurements showed smaller temperature increases (0.17 ± 0.24 °C) and moderate agreement with the reference, whereas rectal (0.12 ± 0.21 °C) and external ear temperatures (0.22 ± 0.38 °C) exhibited higher variability and limited reliability. One cycle was confirmed as anovulatory and correctly identified by the intravaginal sensor.

The most reliable method for detecting ovulation in physically active women is continuous intravaginal temperature measurement. If this method is not available, a trained sublingual temperature measurement at a fixed time can be an acceptable, cost-effective alternative. All methods have their limitations.