Elimination of Amniotic fluid :
Daily fetal urine excretion and fetal swallowing are the best-described processes for AF production and clearance. However, there remain major differences in the magnitude of current estimates of human fetal urine production (36,37,38)
We feel the best estimates of daily amniotic volume flows in the near term fetus are (39)
? Fetal swallowing(Primary source) : Fetal swallowing increases throughout gestation. Although swallowing-type movements can be noted by ultrasound late in the first trimester, these movements do not become well-coordinated until the third trimester. (40,41) Direct and indirect measurements of ovine fetal swallowing suggest that the fetus swallows AF equivalent to 20 to 25 percent of bodyweight 1 The volumes swallowed are significantly greater than in adults, relative to body weight 29. Low rates of swallowing compared to fetal urination in early and mid pregnancy account for the gradual increase in AFV during this period (42) In contrast, increased swallowing near term, and especially postterm, may contribute to the fall in AFV at the end of pregnancy. Fetal swallowing is decreased with a consequent increase in AFV in some fetuses with neurologic abnormalities, such as anencephaly. Fetal swallowing may be also be impaired by obstruction in the fetal esophagus or duodenum.
? Through Respiratory tract : — The fetal lung secretes 100 times as much fluid as is needed to expand the developing lungs to facilitate their growth. The excess fluid exits the trachea, primarily during episodes of fetal breathing (43). Approximately 50percent of this fluid (170 mL/day) is swallowed and the remainder enters the AF compartment, which is the basis for fetal lung maturity testing. (44,45) Lung fluid secretion is reduced during periods of fetal asphyxia.(32) In addition, fetal lung fluid production ceases during labor and fluid is absorbed into the pulmonary lymphatics. In contrast, there are no known circumstances under which fetal lung fluid secretion is increased.
? Intramembrenous Flow : Intramembranous flow is thought to be largely driven by the large osmotic gradient between fetal plasma and AF. Amniotic Fluid may also potentially be removed by continuous bulk flow (i.e. via hydrostatic and oncotic forces). Exchange of fluid may take place at the chorionic plate, where exposure of relative hypotonic amniotic fluid to the fetal surface of placenta may lead to net reabsorption of water by the fetus (up to 8 0ml/day). Transport across the amnion may occur through intercellular channels between amniotic epithelial cells and may be modulated by amniotic fluid prolactin level.
? A final , perhaps underestimated, pathway for volume regulation may occur within the placenta itself. The large surface area of fetal capillary / intervillous interface could magnify small osmolar gradient between a mother and fetus, resulting in large volumes of net water transfer. Exchange of water at this level would influence fetal intravascular volume and potentially affect renal blood flow and urine production. In addition to bulk of fluid, which occurs through pathways that are both phasic (micturition and swallowing) and nonphasic (mediated by hydrostatic and oncotic gradient), there is also bidirectional flow of water between amniotic and maternal compartment. This process occurs by diffusion, but with no net change in fluid volume. At term, water may leave the amniotic cavity at a rate of 400-500ml/hour diffusion plus bulk flow.
? Oral-nasal secretions and transmembranous flow — These pathways account for small volumes of fluid exchange. They are difficult to measure and have not been studied. (32,46) Fetal urine production — 800 to 1200 mL/day
• Fetal lung liquid secretion — 170 mL/day
• Fetal swallowing — 500 to 1000 mL/day
• Intramembranous flow —200 to 500 mL/day
• Oral-nasal secretions — 25 mL/day
• Transmembranous flow —10 mL/day.
Fig.1Amniotic fluid dynamics.(seeds AE: Amniotic fluid physiology. In Sciarra JJ (ed): Gynecology and Obstetrics, Vol 3. New York, &Row,)
NORMAL AMNIOTIC FLUID VOLUME
The amount of amniotic fluid varies with the gestational week at which it is measured. Amniotic fluid volume is most predictable in the first half of pregnancy, when it correspondence with fetal weight.
Week of gestation Fetal weight (gm) Amniotic fluid (ml)
1. 16 100 200
2. 28 1000 1000
3. 36 2500 900
4. 40 3300 800
At 8 weeks the volume increases by around 10ml/week, at 13 weeks the AFI increase by 25ml/week and maximum rate of 60ml/week at 21 weeks gestation. The weekly volume increment then decrease and reaches zero at about 32 weeks of gestation and then starts decreasing at a rate of 8% per week. After 38 weeks, rate of fall in amount of liquor is about 125 ml per week and 150 ml per week after 43 weeks.
AF is about 50 ml at 12 weeks, 400 ml at 20 weeks and 1 liter at 36-38 weeks. The ratio of amniotic fluid volume to fetal weight increase until about 30 weeks of gestation age then appears to decline. It is around 600-800 ml at 40 weeks. As the pregnancy continues post term, further reduction occurs to the extent of 480 ml at 42, 250 ml at 43, and 160 ml at 44 weeks.(46,47)
Fig.2. Amniotic fluid volumes as a function of gestational age. Shaded area occupies 95% confidence interval.(Brace RA, Wolf EJ : Normal amniotic fluid volume changes throughout pregnancy. Am J Obstet Gynecol 161:382, 1989)
? In early pregnancy it is colorless, but near term, it becomes pale straw colored due to presence of exfoliated lanugo and epidermal cells from fetal skin.
? In may look turbid due to presence of vernix caseosa.
? Greenish color : Suggestive of fetal distress/ Meconium in presentations other than breech. It may be thin/thick/pea soup (thick with flakes).
? Golden color : In Rh-incompatibility because of excessive hemolysis of fetal RBC and production of excess bilirubin.
? Saffron – in case of postmaturity.
? Dark brown – In abruptio placenta because of mixture with blood.
? Tobacco juice colored : In intrauterine death because of presence of old HbA.