Menstrual Cycle Temperature Dynamics and Their Association With Conception: A Within- and Between-Person Analysis

Area of Science:

• Reproductive Endocrinology
• Chronobiology
• Fertility Research

Background:

• Menstrual cycle temperature regulation is influenced by reproductive hormones, notably progesterone.
• Basal body temperature tracking is used for ovulation detection, but detailed rhythm characteristics' impact on conception is less understood.
• Individual variability in cycle length and age can influence temperature patterns and conception probability.

Purpose of the Study:

• To investigate the association between menstrual temperature rhythm characteristics (amplitude, phase) and conception likelihood.
• To analyze these associations using both between-person and within-person study designs.
• To account for individual variations in cycle length and age.

Main Methods:

• Analysis of daily temperature data from 423 women (19-40 years) across 4682 menstrual cycles from the 'Fertili' dataset.
• Modeling of cycle-level temperature fluctuations using generalized linear mixed-effects models (GLMMs) with cyclical terms to derive amplitude and phase.
• Within-person comparisons of pre-conceptive and non-conceptive cycles, focusing on cycles with intercourse during the fertile window.

Main Results:

• A robust oscillatory pattern was observed in menstrual temperature across cycles.
• Higher mean temperature and beneficial phase timing were associated with increased conception likelihood, particularly in longer cycles.
• Conception was most likely in shorter cycles (≤35 days) with high amplitude and well-timed phase; in longer cycles, phase timing was more critical.

Conclusions:

• Both the magnitude (amplitude) and timing (phase) of menstrual temperature rhythms provide information about conception potential beyond simple post-ovulatory rise detection.
• Optimal conception likelihood is associated with strong, well-timed temperature rhythms aligning with the fertile window, especially in typical cycle lengths.
• These findings enhance the understanding of fertility markers beyond traditional basal body temperature tracking.