Changes in total body fat during the human reproductive cycle as assessed by magnetic resonance imaging, body water dilution, and skinfold thickness: a comparison of methods.

Total body fat and fat-free mass were assessed by magnetic resonance imaging (TBFMRI and FFMMRI) in 11-16 healthy Swedish women before pregnancy and 5-10 d and 2, 6, and 12 mo after delivery. On these occasions, TBF was also measured by the body water dilution (TBFBWD) and skinfold-thickness (TBFSFT) techniques. The results were used to compare changes in TBFSFT and TBFBWD during reproduction with changes in TBFMRI. TBFBWD was 1.5-4.0 kg higher than TBFMRI and at all postpartum measurements the difference between these estimates increased significantly with increased body fat content. This difference was also significantly higher 6 mo after delivery than it was 2 and 12 mo postpartum. The possibility that this was due to variations in the degree of hydration of FFM postpartum was considered. TBFSFT was 1.7-3.1 kg higher than TBFMRI and this difference increased with increasing body fat content. The agreement between changes in TBFMRI and TBFSFT was different during different times in the reproductive cycle and was also influenced by the amount of fat lost or gained. The findings thus suggest that there is a risk for bias when changes in TBF during reproduction are estimated by the skinfold-thickness technique as well as by isotope dilution.