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Amniotic fluid is in a state of dynamic production and re-absorption.
- Exchange of amniotic fluid occurs through fetal surface of placenta, and fetal skin surface before full keratinization of the skin (approximately at 22 weeks), also fetal respiratory tract and lung with the fetal respiratory movements.
- Production of amniotic fluid is through fetal micturition into the amniotic sac
- Drainage is through fetal swallowing of the amniotic fluid.
Assessment of amniotic fluid volume is reported as early as first trimester scans the amniotic fluid volume matters; it has been found that MSD (mean sac diameter) greater than CRL with less than 5 mm is associated with miscarriage.
Significantly diminished amniotic fluid volume (AFV) in second trimester almost always denotes fetal renal dysgenesis, while if normal in second trimester then starts to decrease in the third trimester may associate placental insufficiency and fetal growth restriction.
Increased amniotic fluid volume may associate cases of spina bifida, fetal GIT obstruction but in most cases it is idiopathic.
Assesment of amniotic fluid volume (AFV) by ultrasound can be done using:
- Single Deepest Vertical Pocket Measurement (SDVP):
With the transducer at right angle to the maternal abdomen the deepest pocket is measured both vertical and transverse, if largest diameter is < 1cm it is considered reduced AFV, if >2cm it is considered normal AFV, if SDVP is > 8cm it is polyhydramnios.
-Amniotic fluid index:
Amniotic fluid index is the objective measure of the subjective assessment of AFV, measure the vertical depth of the largest pocket in every quadrant of the 4 uterine quadrants. The sum of the 4 would be considered:
i. Normal 10 cm – 20 cm
ii. Below average 6 cm – 9 cm
iii. Oligohydramnios < 5 cm
iv. Above average 20 cm -24 cm
v. Polyhydramnios > 24 cm
OR The deepest (maximal) vertical pocket (DVP) depth is considered a reliable method for assessing AFV on ultrasound. It is performed by assessing a pocket of maximal depth of amniotic fluid which is free of UC and fetal parts.
The usually accepted values are:
<2 cm: indicative of oligohydramnious
2-8 cm: normal but should be taken in the context of subjective volume
>8 cm: indicative of polyhydramnious (although some centres use a cut off of >10 cm)