Body composition during normal pregnancy: reference ranges

Maternal body composition undergoes a deep adaptative change during the course of pregnancy. Fat mass, fat-free mass, and total body water (TBW) increase in different ways and their effects on pregnancy outcome represent a field of major interest in perinatal medicine. The aim of this study was to evaluate the changes in maternal body composition [maternal weight, TBW, intracellular water (ICW) and extracellular water (ECW)] during healthy pregnancy by using bioimpedance analysis (BIA). A total of 170 healthy pregnant women, aged 22-44 years, volunteered to participate in our study. The BIA measurements were carried out with a Tefal BIA scale determining resistance and reactance. Lukaski's multiple-regression equation was used to estimate TBW and ICW and ECW were computed using the prediction formula of Segal. The evaluations were performed at 10-38 weeks' gestation, every 3-4 weeks, and hematocrit was determined at every time interval. Analysis of variance and multiple comparisons of Bonferroni were performed to compare variables among the different study intervals. Second-order polynomial interpolation was used to obtain percentile values for each bioimpedance parameter. Percentile bioimpedance values of the healthy population are provided at each study time, by showing the mean value and the 5th, 25th, 75th, 95th percentiles. Moreover, normal reference ranges for TBW are provided for each gestational age, in relation to maternal weight gain. Reactance, TBW, and ICW enhance slightly during the course of gestation. Tetrapolar BIA could be an easy and practical tool for evaluating changes of maternal body components during pregnancy. It could also provide indirect proof of the normal hemodilution occurring in normal pregnancies. Moreover, fat mass deposition, and not only fluid retention, seems to be responsible for the mother's gestational weight gain, since reactance is an indirect parameter in estimating fat mass amount.