✔️Amniotic Fluid -
The precise origin of the liquor amnii is still not well understood. It is probably of mixed maternal and fetal origin.
The water in the amniotic fluid is completely changed and replaced in every 3 hours as shown by the clearance of radioactive sodium injected directly into the amniotic cavity. The presence of lanugo and epithelial scales in the meconium shows that the fluid is swallowed by the fetus and some of it passes from the gut into the fetal plasma (vide scheme).
- Amniotic fluid volume is related to gestational age. It measures about 50 mL at 12 weeks, 400 mL at 20 weeks and reaches its peak of 1 liter at 36–38 weeks.
- Thereafter the amount diminishes, till at term it measures about 600–800 mL. As the pregnancy continues post term, further reduction occurs to the extent of about 200 mL at 43 weeks.
The fluid is faintly alkaline with low specific gravity of 1.010. It becomes highly hypotonic to maternal serum at term pregnancy. An osmolarity of 250 mOsmol/L is suggestive of fetal maturity.
In early pregnancy it is colorless, but near term it becomes pale straw colored due to the presence of exfoliated lanugo and epidermal cells from the fetal skin. It may look turbid due to the presence of vernix caseosa. Abnormal color: Deviation of the normal color of the liquor has got clinical significance.
- Meconium stained (green) is suggestive of fetal distress in presentations other than the breech or transverse. Depending upon the degree and duration of the distress, it may be thin or thick or pea soup (thick with flakes). Thick with presence of flakes suggests chronic fetal distress.
- Golden color in Rh incompatibility is due to excessive hemolysis of the fetal RBC and production of excess bilirubin.
- Greenish yellow (saffron) in post maturity.
- Dark colored in concealed accidental hemorrhage is due to contamination of blood.
- Dark brown (tobacco juice) amniotic fluid is found in IUD. The dark color is due to frequent presence of old HbA.
In the first half of pregnancy, the composition of the fluid is almost identical to a transudate of plasma. But in late pregnancy, the composition is very much altered mainly due to contamination of fetal urinary metabolites. The composition includes—
- water 98–99% and
- solid (1–2%).
The following are the solid constituents-
- Protein–0.3 mg% NPN–30 mg%
- Total lipids–50 mg%
- Glucose–20 mg%
- Uric acid–4 mg%
- Hormones (prolactin, insulin and renin)
- Urea–30 mg%
- Creatinine–2 mg%
(b) Inorganic -
The concentration of the sodium, chloride and potassium is almost the same as that found in maternal blood. As pregnancy advances, there may be slight fall in the sodium and chloride concentration probably due to dilution by hypotonic fetal urine, whereas the potassium concentration remains unaltered.
(c) Suspended particles include-
Lanugo, exfoliated squamous epithelial cells from the fetal skin, vernix caseosa, cast off amniotic cells and cells from the respiratory tract, urinary bladder and vagina of the fetus.
Its main function is to protect the fetus.
- It acts as a shock absorber, protecting the fetus from possible extraneous injury.
- Maintains an even temperature.
- The fluid distends the amniotic sac and thereby allows for growth and free movement of the fetus and prevents adhesion between the fetal parts and amniotic sac.
- Its nutritive value is negligible because of small amount of protein and salt content; however, water supply to the fetus is quite adequate.
- The amnion and chorion are combined to form a hydrostatic wedge which helps in dilatation of the cervix.
- During uterine contraction, it prevents marked interference with the placental circulation so long as the membranes remain intact.
- It guards against umbilical cord compression.
- It flushes the birth canal at the end of first stage of labour and by its aseptic and bactericidal action protects the fetus and prevents ascending infection to the uterine cavity.
- Study of the amniotic fluid provides useful information about the well being and also maturity of the fetus.
- Intra-amniotic instillation of chemicals is used as method of induction of abortion.
- Excess or less volume of liquor amnii is assessed by amniotic fluid index (AFI). Maternal abdomen is divided into quadrants taking the umbilicus, symphysis pubis and the fundus as the reference points.
- Rupture of the membranes with drainage of liquor is a helpful method in induction of labour.