Thermoregulation During Pregnancy: a Controlled Trial Investigating the Risk of Maternal Hyperthermia During Exercise in the Heat

Objectives Despite the well-established benefits of exercise, pregnant women are discouraged from physical activity in hot/humid conditions to avoid hyperthermia (core temperature ( T core ) ≥ 39.0 °C). Recent epidemiological evidence also demonstrates greater risk of negative birth outcomes following heat exposure during pregnancy, possibly due to thermoregulatory impairments. We aimed to determine (1) the risk of pregnant women exceeding a T core of 39.0 °C during moderate-intensity exercise in the heat; and (2) if any thermoregulatory impairments are evident in pregnant ( P ) versus non-pregnant (NP) women. Methods Thirty participants (15 pregnant in their second trimester or third trimester) completed two separate exercise-heat exposures in a climate chamber (32 °C, 45%RH). On separate occasions, each participant cycled on a semi-recumbent cycle ergometer for 45 min at a workload representative of a moderate-intensity (1) non-weight-bearing (NON-WB), or (2) weight-bearing (WB) activity. Thermoregulatory responses were monitored throughout. Results The highest rectal temperature observed in a pregnant individual was 37.93 °C. Mean end-exercise rectal temperature did not differ between groups ( P :37.53 ± 0.22 °C, NP:37.52 ± 0.34 °C, P = 0.954) in the WB trial, but was lower in the P group ( P :37.48 ± 0.25 °C, vs NP:37.73 ± 0.38 °C, P = 0.041) in the NON-WB trial. Whole-body sweat loss was unaltered by pregnancy during WB ( P :266 ± 62 g, NP:264 ± 77 g; P = 0.953) and NON-WB P:265 ± 51 g, NP:300 ± 75 g; P = 0.145) exercise. Pregnant participants reported higher ratings of thermal sensation (felt hotter) than their non-pregnant counterparts in the WB trial ( P = 0.002) but not in the NON-WB trial, ( P = 0.079). Conclusion Pregnant women can perform 45 min of moderate-intensity exercise at 32 °C, 45%RH with very low apparent risk of excessive maternal hyperthermia. No thermoregulatory impairments with pregnancy were observed.