Temperature Regulation in Women and the Effects on the Menstrual Cycle

By: Stephanie Watson

Editor’s Note: More than a hundred years ago, researchers discovered that slight changes in the woman’s core body temperature reflect the various phases of the female cycle. Stephanie Watson, a recent medical school graduate, summarized a 2020 study called Temperature regulation in women: Effects of the menstrual cycle, a review of both current and historical literature designed to describe the characteristics of core body temperature throughout the menstrual cycle. As Watson notes, the study is a complement to lectures and readings presented in  the FACTS elective.

Introduction

The biphasic nature of the woman’s basal core body temperature (BBT) was first described more than 100 years ago [1]. Today, the use of BBT to track the female cycle has been well-established and many women use this sign alone, or in conjunction with their cycle days and/or cervical fluid observations, to confirm when ovulation occurs. The use of BBT with cervical fluid observations is known as the Sympto-thermal method (STM) — one of the most effective fertility awareness-based methods (FABMs), as discussed in the FABMs for Family Planning and Restorative Reproductive Women’s Healthcare elective.

Researchers Baker, Siboza, and Fuller provide a detailed outline of the thermoregulatory system throughout the different phases of the menstrual cycle. The article examines the relationships between core body temperature and cognitive function, sleep, and heart rate. Although body temperature is influenced by numerous external factors, this article focuses on the interaction between BBT and hormones, medications, pregnancy, post-menopause, and environmental exposures.

Methodology

The relationship between the menstrual cycle and thermoregulation was explored through a meta-analysis of peer-reviewed literature. For this analysis, authors selected articles by searching keywords, including, but not limited to, menstrual cycle, body temperature, circadian rhythm, hypothalamus, oral contraceptives, exercise, and heat. The selected literature was reviewed and cross-referenced to ensure representation of different viewpoints of BBT as related to the menstrual cycle. Most studies focused on progesterone and estrogen levels in premenopausal women with regular cycles.

Results and Discussion

The FACTS elective introduces students to BBT within the context of fertility awareness-based methods (FABMs), discussing the role of hormones and their effect on temperature, specifically highlighting how the rise in progesterone results in a concomitant rise in BBT. The article echoes the research on BBT and hormones, citing Moghissi’s “Accuracy of basal body temperature for ovulation detection” chart to indicate that BBT peaks during the luteal phase, concordant with the surge of progesterone. [2] Leading with the scientific “Why?,” the elective then puts into context the “How?” Core body temperature can be a helpful aspect of family planning, something I witnessed firsthand via elective clinical experiences; one patient I observed emphasized the simplicity of using the sympto-thermal method to identify timing of ovulation in order to conceive.

Although many studies reinforce the reliability of the temperature shift in ovulation tracking, the relationship between BBT and hormonal changes remains complicated and nuanced. The FACTS  elective presents research suggesting this biomarker alone may not always be enough to determine when ovulation occurs. [3] An elective guest lecturer highlighted a study on the use of FABMs during the postpartum breastfeeding transition, which found more than half of postpartum women could not determine temperature changes. [4] As such, clinicians may prefer urine LH tests to predict the specific day of ovulation instead of BBT because findings suggest it is more accurate it for women across all reproductive transitions [5].

“The relationship between BBT and hormonal changes remains complicated and nuanced.”

Medications and supplements are another important factor to consider in relation to BBT changes. Physiological effects of supplements on BBT are not well-studied, but the literature  states that melatonin has a generally hypothermic effect. [6] Progesterone and melatonin are inversely related, so that when progesterone is high, the melatonin level is low, which keeps the core body temperature high during the luteal phase and lower in the follicular phase. [6]

Another external factor that can dramatically change the BBT in women is oral contraceptives. Researchers have found that varying oral contraceptive brands and formulations differ in the extent to which they mimic the temperatures in a natural cycle, with different formulations having significant impact on BBT. [1] A study included in the analysis found that progestin-only pills result in a higher core body temperature than combined oral contraceptives, thus concluding that the dominant effect of progestin over estradiol leads to a robust thermogenic effect. Another elective lecturer posited that women should consider stopping hormonal birth control to truly understand their natural cycles and preserve their long-term fertility. It should be noted that some oral contraceptive studies are limited by their comparisons of chronic oral contraceptive users with newer users, potentially leading to inconsistencies in the findings. However, in light of all the research, oral contraceptives not only mask the natural process of the menstrual cycle, but can even cause an imbalance in hormonal regulation and BBT.

Overall, most studies conclude that core body temperature elevation during the luteal phase is affected by much more than hormones. Although progesterone is a major factor causing BBT elevation and can be used to confirm ovulation, medications, reproductive transitions such as pregnancy, post-partum, post-menopause, and environmental exposures remain relevant and important to consider. One strength of this article was the number of resources and literature used to evaluate temperature changes during the menstrual cycle and other influential external factors. This article solidified what the FACTS elective taught me: everyone should have a basic comprehension of the menstrual cycle and the effect of hormonal changes on BBT and other biomarkers; this knowledge will improve their understanding and, ideally, further research, methods, and treatments for women.

Although progesterone is a major factor causing BBT elevation and can be used to confirm ovulation, medications, reproductive transitions such as pregnancy, post-partum, post-menopause, and environmental exposures remain relevant and important to consider.

[1] Baker FC, Siboza F, Fuller A. Temperature regulation in women: Effects of the menstrual cycle. Temperature (Austin). 2020;7(3):226-262. Published 2020 Mar 22. doi:10.1080/23328940.2020.1735927.

[2] Moghissi KS. Accuracy of basal body temperature for ovulation detection. Fertil Steril. 1976;             27:1415–1421.

[3] Barron ML, Fehring RJ. Basal body temperature assessment: is it useful to couples seeking pregnancy?. MCN Am J Matern Child Nurs. 2005;30(5):290-298. doi:10.1097/00005721-200509000-00004

[4] Bouchard T, Fehring RJ, Schneider M. Efficacy of a new postpartum transition protocol for avoiding pregnancy. J Am Board Fam Med. 2013;26(1):35-44. doi:10.3122/jabfm.2013.01.120126

[5] Schliep KC, Mumford SL, Hammoud AO, et al. Luteal phase deficiency in regularly menstruating women: prevalence and overlap in identification based on clinical and biochemical diagnostic criteria. J Clin Endocrinol Metab. 2014;99(6):E1007-E1014. doi:10.1210/jc.2013-3534

[6] Cagnacci A, Soldani R, Laughlin GA, et al. Modification of circadian body temperature rhythm during the luteal menstrual phase: role of melatonin. J Appl Physiol. 1996; 80:25–29.

ABOUT THE AUTHOR

Stephanie Watson

Stephanie Watson graduated in 2022 from Lincoln Memorial University DeBusk College of Osteopathic Medicine in Knoxville, TN. She had a strong interest in holistic women’s health so she participated in a FABM Women’s Health elective during her fall semester of her last year. She looks forward to incorporating FABMs and its endocrine aspects into her future specialty of internal medicine.