Exercise during pregnancy: increased oxygen uptake and metabolic demand.

Current guidelines for exercise during pregnancy are limited in their precision on exercise intensity, as the established compendium for exercise modes and intensities is based on estimates in nongravid adults. To address this gap, we used indirect calorimetry to measure prenatal exercise energy demand during standard aerobic exercises to compare intensity to published standards. Participants (n = 7; 30.7 ± 3.3 yr of age) completed 5 min each of exercise at low and moderate intensity for walking and cycling, on the same testing day, separated by <1 min. For the first 5-min bout, the participant performed an external work rate that elicited low-intensity exercise, i.e., 40%-59% peak aerobic capacity ([Formula: see text]). This was followed by 5 min of the same exercise mode at moderate intensity, i.e., 40%-59% [Formula: see text]. This process was repeated on a cycle ergometer. Measured [Formula: see text] was compared with The American College of Sports Medicine (ACSM)-estimated [Formula: see text] at each trimester. Reported results were presented separately for low and high exercise intensities. Measured absolute and relative [Formula: see text] during walking and cycling exercises were significantly higher (P < 0.001) in all trimesters compared with predicted values, which corresponded with a 35% higher measured metabolic equivalent (MET) value. These findings highlight a critical underestimation in the quantification of effort during prenatal exercise, which is essential to address to safely prescribe exercise for this unique population. ClinicalTrials.gov Identifiers: NCT04805502; NCT06598098.NEW & NOTEWORTHY We measured metabolic equivalents (METs) of prenatal aerobic exercise with indirect calorimetry at light and moderate intensities, then compared with estimates from common equations. Our findings indicate higher oxygen uptake and METs across all trimesters of pregnancy during walk and cycle exercise. We further show an increase in RPE at the same work rate in the second and third trimesters. We conclude that higher metabolic demand must be accounted for in prenatal exercise prescriptions.