The turnover of amniotic fluid protein in the human conceptus

Abstract

The amniotic fluid turnover of specific plasma proteins and protein hormones was studied in 19 pregnant women at 34 to 40 weeks of gestation. Seventeen of the women had normal pregnancies; of these, 6 were in labor during the study, and 11 were not. In the 2 remaining women, the fetuses were dead. Purified human serum albumin, serum γG, serum γA, chorionic gonadotropin, and growth hormone were labeled with either 131I or 125I, and the labeled proteins were then injected intra-amniotically singly or in pairs. Aliquots of amniotic fluid were obtained before the injection, 15 minutes after the injection, and at irregular intervals thereafter during the study period which lasted from 3½ hours to 13 days. Maternal and neonatal sera and urines were also obtained. All fluids were assayed for labeled protein as well as endogenous serum albumin, transferrin, γG, and γA. It was found that: (1) all 5 labeled proteins were cleared from amniotic fluid at similar rates, despite the marked differences in the molecular weights and metabolic functions of these proteins; (2) on the average, two thirds or more of the amniotic fluid volume was cleared of protein per day in the presence of a living fetus, over 80 per cent of this apparently by fetal swallowing, and the daily clearance of amniotic fluid averaged 342 ml. in the absence of labor and 554 ml. during labor, or 0.24 and 0.30 Gm. of amniotic fluid protein per kilogram of fetal weight, respectively; (3) fetal urine was the apparent source of a large fraction of the γG found in amniotic fluid, but fetal urine contributed less than 5 per cent of the albumin, less than 2 per cent of the transferrin, and little or none of the γA present in amniotic fluid; (4) amniotic fluid volume could change markedly in a matter of days—over a period of 5 days, it doubled in one normal patient and fell to half in another; (5) the volume of amniotic fluid swallowed by the fetus tended to vary directly with the volume of fluid in the amniotic cavity, a relation which, among other things, would serve to stabilize amniotic fluid volume.