AMNIOTIC FLUID PHYSIOLOGY AND TESTING
Amnion
Provide a protective cushion for the fetus, allow fetal movement, stabilize the
temperature, permit proper lung development.
Volume of Amniotic Fluid
Regulated by a balance between the production of fetal urine and lung fluid and the
absorption from fetal swallowing and intramembranous flow; approximately 1L.
Polyhydramnios
Excess amniotic fluid from failure of fetus to swallow, associated with neural tube
defects, fetal structural anomalies, cardiac arrhythmias, and congenital infections.
Oligohydramnios
Decreased amniotic fluid from increased fetal swallowing and membrane leakage, can
lead to umbilical cord compression, decelerated heart rate, and fetal death.
Chemical Composition of Amniotic Fluid
Composition similar to maternal plasma with sloughed fetal cells; fetal urine increases
creatinine, urea, and uric acid; glucose and protein concentrations decrease prior to 36
weeks' gestation.
Maternal Urine vs Amniotic Fluid
Creatinine not > 3.5mg/dl and urea not > 30mg/dl; glucose and protein levels differ.
Fern Test
Differentiate amniotic fluid from urine and other body fluids; presence of 'fern-like'
crystals indicates positive screen for amniotic fluid.
Amniocentesis
Procedure to collect amniotic fluid for testing; typically involves 30 ml of fluid.
Indications for Amniocentesis
15-18 weeks of gestation, mother's age of 35 or more at delivery, family history of
chromosome abnormalities, and other specified conditions.
Color and Appearance of Amniotic Fluid
Colorless with slight to moderate turbidity from cells; bright yellow indicates hemolytic
disease, dark green indicates meconium, dark red-brown indicates fetal death.
Tests for Fetal Distress
Includes assessment of HDN, bilirubin levels, and spectrophotometric analysis of fluid
optical density.
Neural Tube Defects
Increased levels of Alpha-fetoprotein (AFP) in maternal blood or amniotic fluid indicate
possible anencephaly or spinal bifida.
Fetal Lung Maturity
Most common complication of early delivery is Respiratory Distress Syndrome;
surfactant decreases surface tension on alveoli.
Lecithin-Sphingomyelin (L/S) Ratio
L/S ratio is 1.6 prior to week 35 and rises to 2.0 or greater for alveolar stability after
week 35; preterm delivery is considered safe with an L/S ratio of 2.0 or higher.
Phosphatidyl Glycerol
Amnion
Provide a protective cushion for the fetus, allow fetal movement, stabilize the
temperature, permit proper lung development.
Volume of Amniotic Fluid
Regulated by a balance between the production of fetal urine and lung fluid and the
absorption from fetal swallowing and intramembranous flow; approximately 1L.
Polyhydramnios
Excess amniotic fluid from failure of fetus to swallow, associated with neural tube
defects, fetal structural anomalies, cardiac arrhythmias, and congenital infections.
Oligohydramnios
Decreased amniotic fluid from increased fetal swallowing and membrane leakage, can
lead to umbilical cord compression, decelerated heart rate, and fetal death.
Chemical Composition of Amniotic Fluid
Composition similar to maternal plasma with sloughed fetal cells; fetal urine increases
creatinine, urea, and uric acid; glucose and protein concentrations decrease prior to 36
weeks' gestation.
Maternal Urine vs Amniotic Fluid
Creatinine not > 3.5mg/dl and urea not > 30mg/dl; glucose and protein levels differ.
Fern Test
Differentiate amniotic fluid from urine and other body fluids; presence of 'fern-like'
crystals indicates positive screen for amniotic fluid.
Amniocentesis
Procedure to collect amniotic fluid for testing; typically involves 30 ml of fluid.
Indications for Amniocentesis
15-18 weeks of gestation, mother's age of 35 or more at delivery, family history of
chromosome abnormalities, and other specified conditions.
Color and Appearance of Amniotic Fluid
Colorless with slight to moderate turbidity from cells; bright yellow indicates hemolytic
disease, dark green indicates meconium, dark red-brown indicates fetal death.
Tests for Fetal Distress
Includes assessment of HDN, bilirubin levels, and spectrophotometric analysis of fluid
optical density.
Neural Tube Defects
Increased levels of Alpha-fetoprotein (AFP) in maternal blood or amniotic fluid indicate
possible anencephaly or spinal bifida.
Fetal Lung Maturity
Most common complication of early delivery is Respiratory Distress Syndrome;
surfactant decreases surface tension on alveoli.
Lecithin-Sphingomyelin (L/S) Ratio
L/S ratio is 1.6 prior to week 35 and rises to 2.0 or greater for alveolar stability after
week 35; preterm delivery is considered safe with an L/S ratio of 2.0 or higher.
Phosphatidyl Glycerol
