Body composition measurement by air displacement plethysmography in pregnancy: Comparison of predicted versus measured thoracic gas volume.

Objectives: Body composition measurements with air displacement plethysmography (ADP) define body volume, which must be corrected for thoracic gas volume (TGV). We hypothesized that physiologic changes owing to pregnancy could affect the accuracy of predicted TGV and introduce errors into body composition measurements.
Methods: We investigated the effect of measuring versus predicting TGV on the accuracy of body composition calculations measured with ADP in overweight and obese pregnant women. The fat and fat-free masses of 110 women were determined with ADP with predicted and measured TGV.
Results: Measured TGV decreased from early to late pregnancy (P = 0.0002). Compared with measured TGV, predicted TGV was 6.3% higher during early gestation and 12.6% higher during late gestation (both P ≤ 0.001). The use of predicted instead of measured TGV in body composition calculations resulted in an overestimation of fat mass by 0.8% during the early stage, and 2.6% during the late stage of pregnancy (both P ≤ 0.001).
Conclusions: Measuring TGV increases the accuracy of body composition measurement by ADP in overweight and obese women, particularly during the late stage of pregnancy.
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