Heat stress and fetal risk. Environmental limits for exercise and passive heat stress during pregnancy: a systematic review with best evidence synthesis

Abstract

ObjectivePregnant women are advised to avoid heat stress (eg, excessive exercise and/or heat exposure) due to the risk of teratogenicity associated with maternal hyperthermia; defined as a core temperature (Tcore) ≥39.0°C. However, guidelines are ambiguous in terms of critical combinations of climate and activity to avoid and may therefore unnecessarily discourage physical activity during pregnancy. Thus, the primary aim was to assess Tcore elevations with different characteristics defining exercise and passive heat stress (intensity, mode, ambient conditions, duration) during pregnancy relative to the critical maternal Tcore of ≥39.0°C.DesignSystematic review with best evidence synthesis.Data sourcesEMBASE, MEDLINE, SCOPUS, CINAHL and Web of Science were searched from inception to 12 July 2017.Study eligibility criteriaStudies reporting the Tcore response of pregnant women, at any period of gestation, to exercise or passive heat stress, were included.Results12 studies satisfied our inclusion criteria (n=347). No woman exceeded a Tcore of 39.0°C. The highest Tcore was 38.9°C, reported during land-based exercise. The highest mean end-trial Tcore was 38.3°C (95% CI 37.7°C to 38.9°C) for land-based exercise, 37.5°C (95% CI 37.3°C to 37.7°C) for water immersion exercise, 36.9°C (95% CI 36.8°C to 37.0°C) for hot water bathing and 37.6°C (95% CI 37.5°C to 37.7°C) for sauna exposure.ConclusionThe highest individual core temperature reported was 38.9°C. Immediately after exercise (either land based or water immersion), the highest mean core temperature was 38.3°C; 0.7°C below the proposed teratogenic threshold. Pregnant women can safely engage in: (1) exercise for up to 35 min at 80%–90% of their maximum heart rate in 25°C and 45% relative humidity (RH); (2) water immersion (≤33.4°C) exercise for up to 45 min; and (3) sitting in hot baths (40°C) or hot/dry saunas (70°C; 15% RH) for up to 20 min, irrespective of pregnancy stage, without reaching a core temperature exceeding the teratogenic threshold.

Effects of prenatal exercise on incidence of congenital anomalies and hyperthermia: a systematic review and meta-analysis

“…Maternal hyperthermia is a well-known teratogen that is associated with congenital anomalies 2–6 51. The findings of our review that prenatal exercise does not exceed hyperthermic thresholds are supported by a recent review 52. The risk of congenital anomalies via hyperthermia during exercise may be attenuated by pregnancy-induced physiological mechanisms that could improve the ability of pregnant women to adequately thermoregulate during and after a bout of exercise.…”

Spotlight on the fetus: how physical activity during pregnancy influences fetal health: a narrative review

“…This is indicative of a transient stress phase for the fetus during exercise. However, this effect may be moderated by the rising maternal body temperature during such exercises74 75 (for more detail, see Ravanelli et al 76).…”

Cardiopulmonary exercise testing during pregnancy

“…A 2019 systematic review and meta‐analysis determined moderate‐to‐vigorous prenatal exercise was not associated with increased odds of congenital abnormalities past the 12th week of gestation (Davenport et al, 2019). In a warm environment (i.e., 25°C with 45% humidity) women are able to safety exercise up to 90% of their heart rate max for 35 minutes (Ravanelli, Casasola, English, Edwards, & Jay, 2019). Thermoregulation during pregnancy is due to a number of cardiac adaptations that improve the body's ability to self‐regulate; This includes elevated cardiac output which supports an increase in blood flow contributing the body's ability to dissipate heat (Perliard, Travers, Racinais, & Sawka, 2016).…”