Amniotic Fluid Physiology and AFI Measurement
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Direct answer
Amniotic fluid physiology describes how liquor is produced, circulated and removed across gestation, and the amniotic fluid index (AFI) estimates its volume. Fetal urine is the main source after the mid-trimester, and fetal swallowing is the main route of removal. The four-quadrant AFI is normal between 5 and 25 cm, and a single deepest pocket below 2 cm indicates oligohydramnios.
What you must remember
- Volume: rises through pregnancy to roughly 1 litre near 34 to 36 weeks, then falls slightly towards term.
- Fetal urine: the main source of fluid after the mid-trimester, with fetal lung fluid and membrane transfer contributing earlier.
- Fetal swallowing: the main route of removal, with fluid absorbed through the fetal gut into the fetal circulation.
- Composition: about 98 to 99 per cent water, with electrolytes close to fetal plasma, plus urea, proteins and phospholipids used in lung maturity testing.
- Normal AFI: 5 to 25 cm by the four-quadrant technique.
- Oligohydramnios: AFI below 5 cm, or a single deepest pocket below 2 cm.
- Polyhydramnios: AFI above 25 cm, or a single deepest pocket above 8 cm.
How the four-quadrant index is measured
The AFI is a semi-quantitative estimate, so technique matters as much as the final number. Examiners often ask for the steps in sequence, so learn them as a routine.
- Position and probe: position the woman comfortably and use a convex transducer over the gravid abdomen, keeping the probe vertical to the ground.
- Divide the uterus: draw a vertical line and a horizontal line through the umbilicus, creating four quadrants.
- Find the deepest pocket: in each quadrant, locate the deepest vertical column of clear fluid, free of umbilical cord and fetal parts.
- Measure each pocket: record the vertical depth of each deepest pocket in centimetres.
- Sum the values: add the four pocket depths to obtain the AFI, then compare the total with the cut-offs.
- Use the single deepest pocket when needed: when the AFI is equivocal, or in settings where a simpler method is preferred, the single deepest pocket provides an alternative estimate.
Where the exam tries to catch you
Questions test the cut-offs and the source of error in measurement, which are different for each technique.
- Trap: counting a loop of umbilical cord as a fluid pocket Correct: exclude the cord and fetal parts, since only clear fluid counts toward the pocket depth.
- Trap: using AFI cut-offs for the single deepest pocket Correct: the AFI is a sum of four pockets with limits of 5 and 25 cm, while the single deepest pocket uses limits of 2 and 8 cm.
- Trap: attributing oligohydramnios to a maternal cause Correct: the main causes are reduced fetal urine from renal agenesis or obstructive uropathy, placental insufficiency, or leakage after rupture of membranes.
Frequently asked questions
What is the normal amniotic fluid index?
The normal amniotic fluid index measured by the four-quadrant technique is between 5 and 25 cm. Values below 5 cm indicate oligohydramnios, while values above 25 cm indicate polyhydramnios.
How is the single deepest pocket measured?
The single deepest pocket is the largest vertical column of amniotic fluid anywhere in the uterus that is free of umbilical cord and fetal parts. A pocket below 2 cm indicates oligohydramnios, and one above 8 cm indicates polyhydramnios.
Where does amniotic fluid come from?
Amniotic fluid comes mainly from fetal urine after the mid-trimester, with fetal lung fluid and membrane transfer contributing earlier in pregnancy. It is removed mainly by fetal swallowing and absorption through the gut, with some flow across the amnion into maternal blood.
What are common causes of oligohydramnios?
Common causes of oligohydramnios include fetal renal agenesis and obstructive uropathy, which reduce fetal urine production. Premature rupture of membranes and placental insufficiency are also important causes, and post-term pregnancy is associated with low fluid volume.
What conditions are linked to polyhydramnios?
Polyhydramnios is linked to maternal diabetes, fetal gastrointestinal anomalies such as oesophageal atresia, and twin-to-twin transfusion syndrome. A detailed anomaly scan is therefore needed whenever polyhydramnios is found.