Objective: In previous studies we have shown that near term pregnant women with idiopathic fetal growth retardation (FGR) have reduced plasma volume (PV) expansion associated with decreased ciruclating levels of placental steroids and aldonsterone (Al do). The objective of the present study was to establish the time course of these changes Study desing In a prospective longitudinal study, 151 normotensive and non-proteinuric pregnant women with a normal weight/height ratio were recruited from those attending two antenatal clinics at Santiago (La Granja and Alejandro del Rio). Since week 10, and at monthly intervals until term, mean arterial pressure (MAP), hematocrit (Ht), PV, Aldo, estradiol (E2) and progesterone (Prog) were measured. At delivery, 23 newborns were considered to have idiopathic FRG Data from these mothers were compared with that obtained in 27 mothers with appropriate fetal growth (C), paired for age, height and parity. Results: Both groups had similar weight gain and MAP throughout gestation. Gestational age at delivery was the same in both groups(39 wks). Newborn's weight was 3575±61 in C and 2712±56 g in the FGR group (p<0.001). Length and ponderal index were significantly reduced in the FGR group. Maternal PV expansion was reduced in mothers with FGR since wk 14 (C=2727±56; FGR=2355±72 ml, p<0.001); this difference was even higher near term (C=3284±68, FGR=2817±91 ml, p<0.001) Ht values were higher in the FGR group throughout gestation. Aldo, E2 and Prog values were significantly reduced in FGR mothers since wk 30. Near term, E2 values were: C=25±3.3, FGR=17.4±1.4 ng/ml (p<0.05); Prog: C=220±22, FGR=162±10.3 ng/ml (p<0.05) and Aldo: C=761±100, FGR=479±56 pg/ml (p<0.05). The prognostic performance of PV measured at wk 14 to predict FGR was: sensitivity and specificity = 71 and 72% respectively, predictive value of the positive test = 38% and of the negative test = 91%.Conclusion: These results indicate that the reduced PV expansion observed in mothers with FGR begins in early pregnancy. This abnormal change cannot be attributed to decreased serum levels of Partially supported by FONDECYT 91-0734.