Body Fluids- Amniotic Fluids

Amniotic fluid

Fluid contained within the amniotic sac surrounding the fetus

Primary function of amniotic fluid

Provides cushioning, allows fetal movement, and enables exchange of water and solutes

Secondary function of amniotic fluid

Prevents umbilical cord compression and protects fetus from trauma

Amniotic fluid formation first trimester

Derived primarily from maternal plasma and fetal transudate

Amniotic fluid formation second trimester

Derived primarily from fetal urine

Amniotic fluid formation third trimester

Maintained by balance of fetal urination and swallowing

Normal amniotic fluid volume first trimester

Approximately 35 milliliters

Normal amniotic fluid volume second trimester

Gradually increases as fetal urine production begins

Normal amniotic fluid volume third trimester

Approximately 1 liter at peak

Amniotic fluid volume near delivery

Decreases prior to labor

Regulation of amniotic fluid volume

Controlled by fetal swallowing and urination

Failure of fetal swallowing

Results in excess amniotic fluid

Excess fetal swallowing

Results in decreased amniotic fluid

Hydramnios

Excessive amniotic fluid volume

Primary cause of hydramnios

Failure of fetal swallowing

Hydramnios associated conditions

Neural tube defects and gastrointestinal obstruction

Oligohydramnios

Decreased amniotic fluid volume

Primary cause of oligohydramnios

Urinary tract defects or membrane rupture

Oligohydramnios complications

Fetal distress and pulmonary hypoplasia

Amniocentesis

Needle aspiration of amniotic fluid

Safe gestational age for amniocentesis

After 14 weeks gestation

Amniocentesis for chromosomal analysis

Typically performed at 16 weeks gestation

Amniocentesis in third trimester

Used to assess fetal lung maturity and distress

Specimen collection precaution

Avoid light exposure to prevent bilirubin degradation

Indications for amniocentesis

Advanced maternal age, genetic screening, fetal maturity, HDN evaluation

Advanced maternal age

Greater than 35 years at delivery

Chromosomal abnormalities detected by amniocentesis

Down syndrome, Turner syndrome, trisomy 18

Inherited disorders detected

Cystic fibrosis, fragile X syndrome, congenital adrenal hyperplasia

X linked disorders detected

Muscular dystrophies

Normal amniotic fluid appearance

Clear to pale yellow

Meconium stained amniotic fluid

Indicates fetal distress

Cloudy amniotic fluid

Suggests infection

Hemolytic disease of the newborn

Immune mediated fetal hemolysis due to blood group incompatibility

Cause of HDN

Maternal antibodies crossing placenta and destroying fetal RBCs

Purpose of amniotic fluid analysis in HDN

Assess severity of fetal anemia

Liley test

Spectrophotometric measurement of bilirubin at 450 nanometers

Principle of Liley test

Measures bilirubin released from fetal RBC destruction

Interpretation of Liley test

Higher delta OD 450 indicates increased hemolysis

Timing of Liley test

Not reliable after 28 weeks due to bilirubin decrease

Interference in Liley test

Light exposure causes falsely low bilirubin results

Specimen handling for Liley test

Do not expose to light

Neural tube defect

Failure of neural tube closure during fetal development

Most common neural tube defects

Spina bifida and anencephaly

AFP

Protein produced by fetal liver

AFP in neural tube defects

Increased in maternal serum and amniotic fluid

Cause of elevated AFP

Failure of skin to close over neural tissue

False positive AFP

Blood contamination of amniotic fluid

False negative AFP

Closed neural tube defects

Acetylcholinesterase

Enzyme present in neural tissue

AChE significance

More specific than AFP for neural tube defects

AChE in amniotic fluid

Increased in open neural tube defects

Fetal lung maturity

Assessment of surfactant production

Surfactant

Phospholipid protein complex that reduces alveolar surface tension

Primary surfactant phospholipid

Lecithin

Secondary surfactant phospholipid

Phosphatidylglycerol

Sphingomyelin

Stable phospholipid used for comparison

Lecithin sphingomyelin ratio

Reference method for fetal lung maturity

Immature L S ratio

Less than 1

5

Borderline L S ratio

Between 1

5 and 2

0

Mature L S ratio

Greater than 2

0

Phosphatidylglycerol significance

Indicates advanced lung maturity

Effect of maternal diabetes on PG

Decreased production despite mature L S ratio

Risk in diabetic mothers

Respiratory distress syndrome despite L S ratio above 2

0

Foam stability test

Assessment of surfactant activity using alcohol

Principle of foam stability test

Surfactant reduces surface tension allowing bubble formation

Positive foam stability test

Stable bubbles indicate lung maturity

Negative foam stability test

Absence of bubbles indicates lung immaturity

Microviscosity test

Measures surfactant to albumin ratio using fluorescence polarization

Albumin during gestation

Remains relatively constant

Surfactant effect on microviscosity

Decreases microviscosity as maturity increases

Mature microviscosity ratio

Greater than 70

Lamellar bodies

Surfactant containing structures secreted by type II pneumocytes

Lamellar body size

Similar to platelet size

Lamellar body count method

Automated cell counter platelet channel

Mature lamellar body count

Greater than 32,000 per microliter

Optical density of amniotic fluid

Increases with lamellar body concentration

Differentiation of urine from amniotic fluid

Based on chemical composition

Creatinine in amniotic fluid

Lower than urine

Urea in amniotic fluid

Lower than urine

Protein in amniotic fluid

Lower than urine

High risk pregnancy

Pregnancy with increased risk of fetal or maternal complications

Examples of high risk pregnancy

Diabetes, Rh incompatibility, advanced maternal age, prior trisomy

Correlation of lab findings and pathology

Used to guide timing of delivery and clinical management

Low surfactant levels

Associated with respiratory distress syndrome

High AFP and AChE

Indicative of open neural tube defect

High bilirubin in amniotic fluid

Indicates severe fetal hemolysis