The changing thermal response to endurance exercise during pregnancy

doi:10.1016/0002-9378(91)90015-J

American Journal of Obstetrics and Gynecology

Volume 165, Issue 6, Part 1, December 1991, Pages 1684-1689

https://doi.org/10.1016/0002-9378(91)90015-J Get rights and content

Abstract

This study was designed to test the hypothesis that the thermal response to endurance exercise is altered by the thermal adaptations to pregnancy. Accordingly, rectal temperature was monitored in 18 recreational athletes before, during, and after 20 minutes of continuous exercise before conception and every 6 to 8 weeks during pregnancy. Mean exercise intensity was 64% of Vo₂ max before conception and did not change during pregnancy. However, the peak rectal temperature reached during exercise decreased by 0.3° C at 8 weeks and then fell at a rate of 0.1° C per lunar month through the thirty-seventh week. This appeared to be related to changes in resting temperature, thermal mass, sweating threshold, and venous capacitance that began early in pregnancy. These data suggest that the magnitude of any exercise-associated thermal stress for the embryo and fetus is markedly reduced by the maternal physiologic adaptations to pregnancy. (AM J OBSTET GYNECOL 1991;165:1684-9.)

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Pregnancy is increasingly considered a period of vulnerability for extreme heat exposure. Multiple lines of evidence support that heat stress is associated with placental insufficiency, poor fetal growth and decreased birth weight. In this narrative review, we first summarize evidence linking ambient temperature or experimentally-induced heat stress with fetal and placental growth outcomes in humans, ruminants and murine species. We then synthesize the literature on putative underlying biological pathways with a focus on the placenta. Reviewed mechanisms include: reduced uterine-placental blood flow, impaired supply of metabolic substrates to the fetus, activation of the maternal stress-response system, and disruption of other endocrine and immune system endpoints. Taken together, this body of evidence supports that exposure to extreme ambient heat likely has adverse consequences for placental development and function. However, research investigating placenta-mediated pathophysiological mechanisms in humans remains extremely limited.
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Citation Excerpt :
The resting rectal temperature decreased as pregnancy progressed, however, from early to late pregnancy, external work (kcal) increased resulting in similar end-exercise sweat production (Jones et al., 1985). A similar study (Clapp, 1991) that assessed thermoregulatory responses to weight bearing exercise (i.e., running) followed 18 physically active pregnant women, the authors ensured participants who maintained exercise equating to 64% VO2peak before conceptionon followed through to late pregnancy by setting the treadmill at a consistent speed/grade and prescribed this exercise intensity between pregnancy timepoints. The onset of sweating was measured subjectively, with participants indicating the moment at which they ‘felt’ sweating start and the associated core temperature was recorded at that time (i.e., thermo-sensitivity) (Clapp, 1991).
The teratogenic risk associated with maternal hyperthermia (i.e., core temperature ≥39.0 °C) has been a crucial motive in understanding thermoregulatory responses in pregnancy. To date, a substantial number of studies have focused on core temperature responses in a wide range of exercise intensities, duration, and ambient temperatures. Fortunately, none have reported core temperatures exceeding 39.0 °C. Nonetheless, there are limited studies that have provided substantial insight into both dry and evaporative heat loss mechanisms involved in facilitating the maintenance of core temperature (≥39.0 °C) during heat stress in pregnant women. The purpose of this scoping review was to summarize the available literature that has assessed heat loss responses in studies of human pregnancy.
A search strategy was developed combining the terms pregnancy, thermoregulation, and adaptation. A systematic search was performed in the following databases: PubMed, Embase, Scopus, and ProQuest. Studies eligible for inclusion included pregnant women between the ages of 18–40 years old, and at least one measurement of the following: sweating, blood flow, skin temperature, and behavioural responses. Retrieved data were categorized as evaporative (sweating), dry or behavioural heat loss responses and summarized narratively.
Thirty-three studies were included in this review (twenty-nine measured physiological responses and four measured behavioural responses). Studies suggest that during exercise, evaporative (sweating) and dry (skin blood flow and temperature) heat loss responses increase from early to late pregnancy in addition to greater cardiac output, blood volume and reduced vascular resistance. Behavioural practices related to heat loss responses are also influenced by cultural/religious expectations, personal preferences and sociodemographics.
The findings from this review suggest that pregnancy modifies evaporative (sweating), dry and behavioural heat loss. However, future studies have an opportunity to compare heat loss measurements accounting for gestational weight gain and thermal sensation/comfort scale to associate physiological responses with perceptual responses across pregnancy.
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These data are supported by Hamilton et al. [113], who demonstrated that greater maternal insulin resistance later in pregnancy, independent of maternal glucose tolerance and parental BMI, was associated with increased infant weight gain and adiposity at one year of age. It has been demonstrated that acute and regular weight-bearing exercise decreases circulating glucose and insulin concentrations in healthy pregnant women [114,115]. Interventions that target a reduction in insulin resistance in the mother may be a mechanism to prevent excessive fat accumulation and/or body mass in the offspring.
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Autonomic mechanisms are fundamental to human physiological thermoregulation, and female reproductive hormones have substantial influences on several aspects of these mechanisms. Of these, the best recognized are the thermoregulatory responses that occur at menopause (hot flushes) and the changes in body temperature within the menstrual cycle which may help couples predict ovulation. Our goal in this brief review is to summarize current knowledge regarding the influences of reproductive hormones on autonomic mechanisms in human thermoregulation. In general, estrogens tend to promote lower body temperatures via augmentation of heat dissipation responses, whereas progesterone tends to promote higher body temperatures. Recent evidence suggests specific influences of estrogens on central autonomic nuclei involved in control of skin blood flow and sweating. Estrogens also augment vasodilation by direct effects on peripheral blood vessels. Influences of progesterone are less well understood, but include both centrally regulated changes in thermoregulatory set-point as well as and peripheral effects, including augmented vasoconstriction in the skin. We conclude with a brief discussion of thermoregulatory adjustments associated with changing hormone levels during menopause, pregnancy and polycystic ovary syndrome.

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: Presented in part at the Thirty-seventh Annual Meeting of the Society For Gynecologic Investigation, St. Louis, Missouri, March 21-24, 1990.

^*: From the Department of Obstetrics and Gynecology, University of Vermont College of Medicine.

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The changing thermal response to endurance exercise during pregnancy

Abstract

Am J Obstet Gynecol

Am J Obstet Gynecol

Am J Obstet Gynecol

Lancet

Am J Obstet Gynecol

Am J Obstet Gynecol

Thermoregulatory and metabolic responses to jogging prior to and during pregnancy

Med Sci Sports Exerc

Oral and rectal temperatures in runners

Phys Sportsmed

Sexual variations in thermoregulation during heat stress

Aviat Space Environ Med