3.4. Basal metabolic rate (BMR)

Alterations in BMR could account for a considerable proportion of the total theoretical cost of pregnancy. It has also been suggested that adaptations in BMR might result in energy savings. In each centre, BMR was measured using the Douglas Bag technique of indirect calorimetry. The women were studied in the resting, fasted state first thing in the morning and, after half to three-quarters of an hour of lying in bed in a quiet environment, two measurements were done, the first lasting 15 minutes and the second, 10.

The volume of expired air was measured using a calibrated air meter, and the O₂ and CO₂ contents of the expired air were obtained by the Servomex 570A paramagnetic oxygen analyser and the PK Morgan infra-red CO₂ analyser, both of these instruments also being frequently calibrated against known gas mixtures. Agreement for the duplicate measures of BMR was expected to be within 3%; if this was not the case, a third measurement was done and the mean value of all three was taken as the BMR. BMR was measured in the laboratory, to obtain quietness with a minimum of extraneous distractions.