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

Temperature regulation across the menstrual cycle follows predictable rhythmic changes driven by reproductive hormones, particularly the thermogenic effect of progesterone in the luteal phase. While basal body temperature has long been used to identify ovulatory cycles, it is less clear how detailed features of the temperature rhythm, including its strength (amplitude) and timing (phase), relate to the likelihood of conception, especially when accounting for individual variability in cycle length and age. Here, we aimed to examine associations between menstrual temperature rhythm characteristics and conception likelihood using both between-person and within-person analyses. We analyzed daily temperature data from 423 women (19-40 years) contributing 4682 cycles, who were participants in a multi-country study about fertility conducted between 1992 and 1996 ("Fertili" dataset). Cycle-level temperature fluctuations were modeled using linear mixed-effects models (GLMMs) with cosine and sine terms scaled to each cycle's length, from which amplitude and phase parameters were derived. At the level of consecutive cycle-series (sessions), GLMMs assessed whether rhythm features, mean temperature, cycle length, cycle regularity, and age predicted conception. Within-person analyses compared pre-conceptive and non-conceptive cycles from the same individual, restricted to cycles with sexual intercourse during the fertile window. Temperature showed a robust oscillatory pattern across the menstrual cycle. At the session level, higher mean temperature was associated with greater conception likelihood in the pre-conceptive cycles, and phase in temperature rhythms tended to be beneficial, particularly in longer cycles. A 3-way interaction revealed that conception was most likely in cycles following shorter cycles (≤35 days) when temperature rhythms were both high in amplitude and well-timed in phase, whereas in longer cycles, rhythm timing appeared to play a larger role than amplitude alone. Within-person comparisons showed that larger temperature phase occurred more often in pre-conceptive cycles than in cycles not followed by conception. Both the magnitude and timing of menstrual temperature rhythms carry information about potential for conception beyond the detection of a post-ovulatory rise. Conception appears most likely when strong rhythmicity aligns optimally with the fertile window in typical length cycles.