Amniotic Fluid Index and Single Deepest Pocket
Amniotic Fluid Index and Single Deepest Pocket
Two published methods for the same volume, and they disagree systematically. Five randomised trials found the index diagnoses oligohydramnios more than twice as often as the pocket without improving the neonatal outcome. Enter both and see which side you are on.
Both methods, side by side
The disagreement is the pointFour quadrant pockets of 0.3, 0.3, 0.6 and 2.6 cm: an amniotic fluid index of 3.8 cm and a single deepest pocket of 2.6 cm
The two methods, and the evidence between them
Low: AFI below 5 cm · SDP below 2 cm — and the index calls it 2.39 times as often
- AFI below 5 cm
- the oligohydramnios threshold for the four-quadrant index. The normal range is quoted as 5 to 25 cm. An index of 0 cm is anhydramnios
- SDP below 2 cm
- the oligohydramnios threshold for the single deepest pocket, with a normal range of 2 to 8 cm. This is the threshold ACOG supports, and it is also the amniotic fluid component of the biophysical profile, where the criterion is a pocket of 2 cm in two perpendicular planes
- the direction of the disagreement
- the index diagnoses oligohydramnios MORE OFTEN than the pocket does, not less. Risk ratio 2.39 across five randomised trials and 3,226 women, with 1.92 for induction of labour and 1.46 for caesarean delivery for suspected distress, and no difference in neonatal intensive care admission or acidaemia
- which guidance says what
- ACOG supports a deepest vertical pocket of 2 cm or less rather than an index of 5 cm or less, on the basis of the randomised trials. The Cochrane authors reached the same conclusion and also called for a systematic review of the diagnostic accuracy of both methods, which is a reminder that the comparison above is of OUTCOMES, not of accuracy against a dye-dilution volume
- the upper thresholds are not standardised
- 24 cm and 25 cm are both published for the index and 8 cm and 10 cm for the pocket, so a given measurement can be polyhydramnios under one convention and normal under another. Both are offered above rather than one being chosen for you
Worked example
Four quadrant pockets of 0.3, 0.3, 0.6 and 2.6 cm: an amniotic fluid index of 3.8 cm and a single deepest pocket of 2.6 cm
AFI = 0.3 + 0.3 + 0.6 + 2.6 = 3.8 cm, which is below the 5 cm threshold
SDP = max(0.3, 0.3, 0.6, 2.6) = 2.6 cm, which is above the 2 cm threshold
So the index calls oligohydramnios and the pocket does not. That is the direction the randomised evidence found, and it is why ACOG supports the pocket
Redistribute the same total evenly as four 0.95 cm pockets and both methods now call it low
Four pockets of 1.5 cm give an index of 6.0 cm, which is normal, while every pocket is under 2 cm — the opposite disagreement, and the page reports it separately
The two methods and their published thresholds
| Amniotic fluid index | Single deepest pocket | |
|---|---|---|
| What is measured | The sum of four quadrant pockets | One cord-free and limb-free pocket |
| Oligohydramnios | Below 5 cm | Below 2 cm |
| Normal range | 5 to 25 cm | 2 to 8 cm |
| Polyhydramnios | Above 24 cm, or above 25 cm | Above 8 cm, or above 10 cm in some centres |
| Anhydramnios | 0 cm | No measurable pocket |
| Which ACOG supports for oligohydramnios | Not this one | This one, on the randomised evidence |
What the five randomised trials found
| Outcome | Risk ratio with the index | 95% interval |
|---|---|---|
| Diagnosis of oligohydramnios | 2.39 | 1.73 to 3.28 |
| Induction of labour | 1.92 | 1.50 to 2.46 |
| Caesarean delivery for suspected fetal distress | 1.46 | 1.08 to 1.96 |
| Admission to neonatal intensive care | 1.04 | 0.85 to 1.26 |
| Neonatal acidaemia, umbilical artery pH below 7.1 | No difference reported | Not stated |
| Non-reassuring tracings, meconium, 5-minute Apgar below 7 | No difference reported | Not stated |
Why two methods for one volume, and which way the disagreement runs
Both numbers are ultrasound surrogates for amniotic fluid volume and neither measures it. The four-quadrant index sums the deepest vertical pocket in each quadrant; the single deepest pocket takes one measurement of the deepest cord-free and limb-free pool. Because the index is a sum containing the deepest pocket, it can never be smaller than it — but the ratio between them depends entirely on how evenly the fluid is distributed, and that is where the two methods part company.
The direction of the disagreement is known and it is one-sided. Across five randomised trials and 3,226 singleton pregnancies, using the index diagnosed oligohydramnios 2.39 times as often as using the pocket. It also produced 1.92 times as much induction of labour and 1.46 times as many caesarean deliveries for suspected fetal distress. Admission to neonatal intensive care was unchanged (risk ratio 1.04, interval 0.85 to 1.26), as were neonatal acidaemia, non-reassuring tracings, meconium and the five-minute Apgar score. More diagnoses, more intervention, the same outcomes. That is why the Cochrane authors concluded the pocket is the better choice and why ACOG supports a deepest vertical pocket of 2 cm or less rather than an index of 5 cm or less. The title of one of the influential papers in the field is literally the evidence for abandoning the index in favour of the pocket.
Three things this page deliberately does not do. It does not pick a method for you: it applies both and names the guidance, because a unit that reports an index is not going to stop on the strength of a web page. It does not hold a centile. Moore and Cayle’s percentile curves, from 791 normal pregnancies, put the index at term at a mean of 115 mm with a 5th-to-95th range of 68 to 196 mm, and their own conclusion is that the value differs significantly from week to week and must be read against week-specific tables — which this engine cannot interpolate, so it does not pretend to. And it renders no management: the amniotic fluid volume is one component of the biophysical profile and of the modified profile, it is read with the growth-restriction criteria and the umbilical artery Doppler, and it is read against a gestational age the dating pages establish. This page computes a number, or applies published criteria, and names the body that publishes them. It renders no clinical decision: nothing here says whether this fetus is compromised, whether a tracing should be acted on, whether a scan should be repeated, whether labour should be induced or when a pregnancy should be delivered. An obstetric measurement is read alongside the whole clinical picture – history, gestational age, serial trend, the other biometry – and never instead of it. It supports a clinician’s judgement rather than replacing it. Thresholds here are the ones their named sources print. Reference standards differ between guidelines, a finding can cross a threshold purely by changing standard, and the cut-off your own unit works to takes precedence over anything on this page.
Frequently asked questions
Which method should a unit use?
ACOG supports the single deepest vertical pocket of 2 cm or less rather than an amniotic fluid index of 5 cm or less for diagnosing oligohydramnios, on the basis of the randomised trials, and the Cochrane authors reached the same conclusion. The reason is that the index finds more oligohydramnios without finding more compromised babies. This page states that guidance and applies both methods; the decision belongs to your unit’s protocol.
Can the index be low while the pocket is normal?
Yes, and it is the commoner disagreement. Four quadrant pockets of 0.3, 0.3, 0.6 and 2.6 cm sum to 3.8 cm, which is below the 5 cm threshold, while the deepest single pocket is 2.6 cm and well above 2 cm. Uneven distribution does this. The reverse also happens: four even pockets of 1.5 cm give an index of 6 cm with every pocket under 2 cm.
Why does the page ask which upper cut-off to use?
Because both are published and they are different thresholds. An index above 24 cm and an index above 25 cm are both quoted for polyhydramnios, and a value of 24.5 cm is raised on one and normal on the other. For the pocket, 8 cm is the top of the usual 2-to-8 cm range while some centres, particularly in Australia, New Zealand and the UK, use 10 cm.
Why is there no centile for gestational age?
Because the published centiles are week-by-week tables and this calculator cannot interpolate one. Moore and Cayle measured 791 normal pregnancies and built a mean and a 90% interval for each week, and their conclusion was that the values differ significantly between weeks. A curve fitted to that table would misclassify at the extremes, which is exactly where the decision sits, so none is fitted. Read the number against your own unit’s chart for the gestational age.
Is an abnormal result a diagnosis?
No. Both numbers are surrogates for a volume neither measures, with interobserver variation around 3% and intraobserver variation around 7% in Moore and Cayle’s series, and the randomised evidence shows that acting on the more sensitive of the two does not improve the neonatal outcome. The finding is read with the gestational age, the growth, the Dopplers and the clinical picture.
Related calculators
References
- Nabhan AF, Abdelmoula YA. Amniotic fluid index versus single deepest vertical pocket as a screening test for preventing adverse pregnancy outcome. Cochrane Database Syst Rev. 2008;(3):CD006593. doi:10.1002/14651858.CD006593.pub2
- Perinatology.com. Amniotic Fluid Index (glossary), citing Rutherford SE, et al. Obstet Gynecol. 1987;70:353-6; Phelan JP, et al. J Reprod Med. 1987;32:540-2; and American College of Obstetricians and Gynecologists, Antepartum Fetal Surveillance, Practice Bulletin No. 145. Obstet Gynecol. 2014;124:182-92.
- Moore TR, Cayle JE. The amniotic fluid index in normal human pregnancy. Am J Obstet Gynecol. 1990;162(5):1168-73.
- Radiopaedia. Deepest vertical pocket method, citing Magann EF, et al. Ultrasound Obstet Gynecol. 2002;20(5):464-7.
- Magann EF, Chauhan SP, Doherty DA, Magann MI, Morrison JC. The evidence for abandoning the amniotic fluid index in favor of the single deepest pocket. Am J Perinatol. 2007;24(9):549-55.
Not medical advice. For healthcare professionals and education. Reference intervals vary by laboratory and assay — always use your own laboratory's. Never base a dose or a treatment decision on this page alone. Full disclaimer at calcengines.com/disclaimer/