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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
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Much more is known about the pathways involved in the regulation of AVF during the second half of gestation after the fetal skin keratinizes. The excretion of fetal urine and the swallowing of amniotic fluid by the fetus are the two major pathways for the formation and clearance, respectively, of amniotic fluid. 1. Fetal urine is a major source of amniotic fluid in the second half of pregnancy. Urine first enters the amniotic space at 8-11 weeks’ gestation, but the urine production rate shows a steady increase throughout the last half of gestation (3). Urine production per kilogram of body weight increases from approximately 110/ml/kg every 24 hours at 25 weeks to approximately 190 ml/kg every 24 hours at 39 weeks (4). At term, the current best estimate of fetal urine flow rate may average 700-900 ml/day. 2. The fetus begins swallowing at the same gestational age when urine first enters the amniotic space, that is around 8-11 weeks. It is estimated that the volume of amniotic fluid swallowed in late gestation averages 210-760 ml/day (5), and that this process primarily occurs during episodes of fetal breathing activity. These daily volumes do not include the amount of tracheal fluid from the lungs that is swallowed before it enters the amniotic space. Fetal swallowing plays an important role

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