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ACTA BIO MEDICA ATENEO PARMENSE 2004; 75; Suppl. 1: 11-13

© Mattioli 1885

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Amniotic fluid dynamics
Alberto Bacchi Modena, Stefania Fieni
Department of Obstetrics, Gynecology and Neonatology. University of Parma

Abstract. Amniotic fluid was once considered to be a stagnant pool, approximately circulating with a turnover time of one day.Adequate amniotic fluid volume is maintained by a balance of fetal fluid production (lung liquid and urine) and resorption (swallowing and intramembranous flow). Even though different hypotheses have been advanced on the mechanisms regulating this turnover, the inflow and outflow mechanism that keeps amniotic fluid volume within the normal range is not entirely clear. Regulatory mechanisms act at threelevels: placental control of water and solute transfer; regulation of inflows and outflows from the fetus; and, maternal effect on fetal fluid balance. Amniotic fluid is 98-99% water. The chemical composition of its substances varies with gestational age. When fetal urine begins to enter the amniotic sac, amniotic osmolarity decreases slightly compared with fetal blood. After keratinization of thefetal skin, amniotic fluid osmolarity decreases further with advancing gestational age. The low amniotic fluid osmolarity, which is produced by the inflow of markedly hypotonic fetal urine, provides a large potential osmotic force for the outward flow of water across the intramembranous and transmembranous pathways. Within certain limits, amniotic fluid mirrors the metabolic status of thefetoplacental unit; for that reason, a study of its components and their respective variations in the different weeks of pregnancy provides useful indications, both for a correct assessment of fetal maturation and for an evaluation of kidney function parameters and placental insufficiency. Key words: Amniotic fluid, physiology, regulatory mechanisms

Amniotic fluid physiology About 4 liters of wateraccumulate within intrauterine compartments during the 40-week period of human gestation, with 2800 ml in the fetus, 400 ml in the placenta, and 800 ml in the amniotic fluid. At the beginning of pregnancy, amniotic fluid volume (AFV) is a multiple of fetal volume. The two volumes become equal soon after the 20th week, but by the 30th week AVF is about half the fetal volume and at term it is about aquarter of fetal volume. In the last trimester, near term, there are net increases of about 30 to 40 ml per day. Such daily net increases in intrauterine fluid exceed the amount that could originate from any intrauterine or fetal metabolic source, and thus must in large part be derived by transfer from the maternal

compartment. Human placental tissue, a multicellular tissue layer with largeextracellular spaces, reflects the physicochemical properties of a membrane system with significant porosity. In normal pregnancies, a wide volume range is seen, particularly during the second half of gestation. The rate of change in AVF is a strong function of gestational age. There is a progressive AFV increase from 30 ml at 10 weeks’ gestation to 190 ml at 16 weeks and to a mean of 780 ml at 32-35weeks, after which a decrease occurs. The decrease in post-term pregnancies has been found to be as high as 150 ml/week from 38 to 43 weeks (1). Although the basic mechanisms that produce these AVF changes throughout gestation are unclear, it is important to note that, when expressed as a percentage, the rate of chan-

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A. Bacchi Modena, S. Fieni

ge decreases monotonically throughoutthe fetal period. Thus, the late decrease in AVF represents a natural progression rather than an aberration. AVF is the sum of the inflows and outflows in the amniotic space. Thus, knowledge of the pathways for amniotic fluid movements is a prerequisite for understanding volume regulatory mechanisms (2). In early gestation, significant amounts of amniotic fluid are present before the establishment...