Amniotic Fluid (Textbook) Flashcards
Amniotic Fluid Components
Present in the amnion, a membranous sac that surrounds the fetus
Amnion is metabolically active and is involved in the exchanges of water and chemicals between the fluid, the fetus, and the maternal circulation
Produces: peptides, growth factors, and cytokines
Regulated by: production of fetal urine (fetus swallows amniotic fluid and regulates the increase in fluid from the fetal urine) and lung fluid vs absorption from fetal swallowing and intramembranous flow.
Amniotic Fluid Functions
- Protective cushion for the fetus
- Allow fetal movement
- Stabilize the temperature
- Permit proper lung development
Intramembranous flow
Absorption of amniotic fluid water and solutes into the fetal vascular system
Amniotic Fluid Volumes
Polyhydramnios: volume greater than 1200 mL, fetus fails to begin swallowing amniotic fluid (indicating fetal distress/neural tube disorders)
Oligohydramnios: volume less than 800 mL, fetus increases amniotic fluid swallowing or urinary tract deformities/membrane elakage are present
Fetal Respiration
Fetus secretes a volume of lung liquid necessary to expand the lungs with growth.
During each episode of fetal respiratory movement, secreted lung liquid enters the amniotic fluid, bathing the lungs and washing lecithin, sphingomyelin, and phosphatidyl glycerol into the amniotic fluid surrounding the fetus
Physical and Chemical Tests of Amniotic Fluid
Fetal squamous cells for cytogenetic analysis.
Biochemical substances: bilirubin, lipids, enzymes, electrolytes, urea, creatinine, uric acid, proteins, and hormones
Markers for Neural Tube Defects
Alpha-fetoprotein (major protein produced by the fetal liver during early gestation) and acetylcholinesterase are two biochemical markers tested for these defects.
Tests for Fetal Urine Production
Concentrations of creatinine, urea, and uric acid increase
Glucose and protein concentrations decrease
If there is concern of contamination, levels of creatinine and urea are much lower in amniotic fluid than in urine.
Amniocentesis
Screening for neural tube defects when blood tests such as the maternal serum alphafetoprotein test are abnormal, to detect genetic disorders, or to evaluate the health of the fetus
A thin, hollow needle is inserted through the mother’s abdomen into the uterus and the amniotic sac to aspirate the amniotic fluid.
Cytogenetic Tests
Cells can be separated from the fluid, cultured, and examined for chromosome abnormalities by:
- Karyotyping
- Fluorescence in situ hybridization (FISH)
- Fluorescent mapping spectral karyotyping (SKY)
- DNA testing
Meconium
Newborn’s first bowel movement, is formed in the intestine from fetal intestinal secretions and swallowed amniotic fluid
Liley Graph
Used to determine the severity of the hemolytic disease based on spectrophotometric readings of bilirubin versus fetal age
Specimens contaminated with meconium will cause falsely low values, blood’s maximum absorbance of oxyhemoglobin occurs at 410 nm and can interfere with the bilirubin absorption peak
Interference can be removed by extraction with chloroform if necessary.
Multiples of the Median (MoM)
- All AFP levels are reported in this form.
- The median is the reference level for a given week of gestation.
- A value two times the median value is considered abnormal (greater than 2 MoM) for both maternal serum and amniotic fluid. Elevated amniotic fluid AFP levels are followed by measurement of amniotic acetylcholinesterase (AChE).
Respiratory Distress Syndrome (RDS)
Caused by an insufficiency of lung surfactant production and structural immaturity of the fetal lungs.
Surfactant keeps the alveoli from collapsing by decreasing surface tension and allows them to inflate with air more easily.
Lecithin-Sphingomyelin Ratio (L/S)
Lecithin is the primary component of the surfactants, produced at a low and constant rate until the 35th week of gestation when it increases greatly, stabilizing the fetal lung alveoli.
- Sphingomyelin serves as a control on which to base the rise in lecithin as it starts forming earlier
- A rise of 2.0 is considered acceptable for preterm delivery
Phosphatidyl Glycerol (PG)
Needed for adequate lung maturity and parallels prodution of lecithin
Can be found through thin-layer chromotography or an immunoagglutination test
Foam Test
- A mechanical screening test
- Amniotic fluid is mixed with 95% ethanol, shaken for 15 seconds, and allowed to sit undisturbed for 15 minutes.
- At the end of this time, the surface of the fluid is observed for the presence of bubbles, indicating that a sufficient amount of phospholipid is available to reduce the surface tension
Lamellar Body Count
- Lamellar bodies are densely packed layers of phospholipids that represent a storage form of pulmonary surfactant.
- As the fetal lung matures, increased lamellar body production increases OD, as well as being reflected by an increase in amniotic fluid phospholipids and the L/S ratio.
- Reported as # Lamellar Bodies/µL
Automated Lamellar Body Count
Counts can be obtained using the platelet channel of automated hematology analyzers using either optical or impedance methods for counting
Blood raises the LBC because of platelet presence, then lowers it as lamellar bodies are trapped in fibrin strands.
