9 Because amniotic fluid is more hypotonic than fetal plasma, it is postulated that exposure of amniotic fluid to the fetal alveolar capillary bed results in net movement of water from the amniotic cavity into the fetus. 8 At term, inspiratory flow in the fetus is approximately 200 ml/kg/day, up to 600–800 ml/day. Fetal respiratory activity has been observed as early as 11 weeks' gestation. This fluid is absorbed through the fetal gastrointestinal system and is either recycled through the kidneys or is transferred to the maternal compartment through the placenta.Ī second, more debatable means of amniotic fluid removal may be by the respiratory tract. 7 Studies using radiolabeled red blood cells and radioactive colloid estimate that, on average, a fetus swallows from 200 to 450 ml/day at term, removing 50% of the amniotic fluid produced through fetal urination. The primary source of elimination is through fetal swallowing, which has been observed as early as 16 weeks. 4, 5, 6Īmniotic fluid is eliminated by at least three mechanisms.
By term, a fetus produces on average from 500 to 700 ml/day with a slight decline in hourly fetal urine production after 40 weeks' gestation. 3 Because fetal urine is hypotonic (80–140 mOsm/ liter), it results in progressively hypotonic fluid (250–260 mOsm/liter near term) that contains increasing concentrations of urea, uric acid, and creatinine as the fetal kidneys mature. Urine has been observed in the fetal bladder as early as 11 weeks transabdominally 2 and 9 weeks transvaginally.
At this time, a fetus contributes to amniotic fluid volume and composition almost exclusively through urination.
#Low amniotic fluid at 38 weeks skin#
Active secretion of fluid from the amniotic epithelium had been previously suggested to play a role in early amniotic fluid formation, but this has not been demonstrated.īy the second trimester, the fetal skin becomes keratinized, making it impermeable to further diffusion. 1 Thus amniotic fluid in early gestation is a dialysate that is identical to the fetal and maternal plasma, but with a lower protein concentration. Water and solutes freely traverse fetal skin and may diffuse through the amnion and chorion as well. In the first half of pregnancy, amniotic fluid is derived from fetal and possibly maternal compartments. 1), discusses the causality and perinatal significance of volume disturbances, and reviews the techniques of ultrasonographic assessment of amniotic fluid volume, as well as their role in the antenatal testing of high-risk fetuses. This chapter reviews the dynamics of amniotic fluid volume (Fig. Because precise quantification of amniotic fluid volume is not possible with ultrasonography, various techniques for both qualitative and semiquantitative assessment have been proposed. With the advent of real-time ultrasonography, assessment of amniotic fluid has been possible, resulting in earlier recognition of abnormal conditions and possible intervention. These disorders may result from abnormal fetal or maternal conditions and, conversely, may be responsible for alterations of fetal well-being as well. Disorders of this regulatory process can lead to either polyhydramnios or oligohydramnios, in which too much or too little fluid exists, respectively. The quantity of amniotic fluid at any time in gestation is the product of water exchange between the mother, fetus, and placenta, and is maintained within a relatively narrow range. In addition, its bacteriostatic action helps prevent infection of the intra-amniotic environment. It cushions the fetus from injury, helps prevent compression of the umbilical cord, and allows room for it to move and grow. Amniotic fluid is vital to the well-being of the fetus.
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