Assessment of change in hydration in women during pregnancy and postpartum with bioelectrical impedance vectors

Objective This study tested the hypothesis that bioelectrical impedance vectors, group and individual, are valid indicators of total body water (TBW) and hydration status in women experiencing fluid gain and loss during and after pregnancy. Methods We measured TBW, assessed with D2O dilution, and resistance (R) and reactance (Xc), determined with 800 μA at 50 kHz and standardized for height (H) and plotted on a bivariate (R-Xc) graph, in 15 women, 21–37 y of age, longitudinally before and during pregnancy and postpartum (PP). Results Body weight (61.9 ± 2.3 to 75.5 ± 2.3 kg) and TBW (31.4 ± 1.1 to 38.2 ± 1.1 L) increased (P < 0.05) from before pregnancy to the third trimester of pregnancy and decreased PP (67.0 ± 2.3 kg and 32.7 ± 1.1 L, P < 0.05). R/H and Xc/H decreased during pregnancy (P < 0.05, 361 ± 10 to 318 ± 10 and 44 ± 1 to 36 ± 1 ω/m, respectively) and increased PP (P < 0.05, 355 ± 10 and 41 ± 1 Ω/m). Vector length decreased (P < 0.05, 363 ± 10 to 320 ± 10 Ω) during pregnancy and increased PP (P < 0.05, 357 ± 10 Ω). Changes in vector length and TBW during pregnancy and PP were correlated (r = −0.599, P < 0.001). Women with vectors exceeding a 75% tolerance interval had greater TBW gain (10–12 versus 5–6 L) during pregnancy compared with other women with vectors within this tolerance level. Conclusion These findings indicate that impedance vectors provide quantitative evidence of hydration status during pregnancy and that the impedance vector method is useful in monitoring hydration status in pregnancy.