Résumé : Coelomic fluid (n = 57), amniotic fluid (n = 61) and maternal serum (n=81) were obtained from normal pregnancies between 7.7 and 13.9 weeks and assayed for total protein, α-fetoprotein (αFP), albumin and pre-albumin. The mean concentration of total protein in matched samples was 18 times higher in maternal serum than in the coelomic fluid and 54 times higher in the coelomic fluid than in amniotic fluid. The concentrations of total protein, albumin and pre-albumin decreased and that of αFP increased in maternal serum with advancing gestation. The yolk sac volume and the concentrations of total protein in the coelomic and amniotic fluids increased with gestational age. No difference was found for the crown-rump length, yolk sac volume and protein concentration in the coelomic fluid between two groups presenting with low and high maternal serum pre-albumin concentrations. Before 11 weeks gestation, significant correlation was only found between yolk sac volume and coelomic fluid concentration of pre-albumin as evaluated by both electrophoresis and immunonephelometry. These results suggest that during the first trimester of normal pregnancy, the placental metabolism and transfer rate of proteins is not directly influenced by the concentrations of protein in the maternal circulation and that the transfer of proteins through the amniotic membrane is limited. These results also indicate that during that period the secondary yolk sac may contribute to the protein content of the exocoelomic cavity, and that the embryo and its yolk sac and subsequently the fetus are the main source of the proteins present in the amniotic fluid. © 1994 Oxford University Press.