BACKGROUND: Knowledge of the biological variability of the hydration factor (HF), ie, the ratio between total body water and fat-free mass, is important when calculating total body fat by means of the commonly used two-component model, which is based on estimates of body weight and total body water. The effect of pregnancy on the biological variability of HF, and consequently on the precision of the two-component model, is unknown. OBJECTIVE: Our goal was to assess the effect of pregnancy on HF and its biological variability. DESIGN: HF was assessed in 33 women planning pregnancy and in 17 of these women during gestational weeks 14 and 32 and 2 wk postpartum. HF was calculated by using estimates of body weight, total body water obtained by means of deuterium dilution, and body volume measured by using underwater weighing. RESULTS: In the 17 women who became pregnant, HF was 0.718 +/- 0.023, 0.723 +/- 0.031, 0.747 +/- 0.017, and 0.734 +/- 0.020 before pregnancy, in gestational week 14, in gestational week 32, and 2 wk postpartum, respectively. The biological variability represented approximately 2% of average HF in the nonpregnant state. The corresponding figure was >3% in gestational week 14 but