Amniotic fluid disorders — polyhydramnios and oligohydramnios

A registrar's first diagnosis of "polyhydramnios" on a growth scan is rarely an endpoint — it is the start of a hunt. The same is true of oligohydramnios. Both are signs, not diseases, and the examiner wants to see that you can take either finding, frame it as a measurement with a threshold, work through a structured differential, and act on the cause rather than the number. This topic is that framework.^[2][10]

Overview and definition

Amniotic fluid volume is a vital sign of the fetus, and like any vital sign it has a number you can quote and a method you can defend.^[2]

• Polyhydramnios — an abnormal increase in fluid. By ultrasound, a deepest vertical pocket (DVP) of 8 cm or more, or an amniotic fluid index (AFI) of 24 cm or more in a singleton (SMFM); by true volume, more than 2000 mL.^[1][2]
• Oligohydramnios — an abnormal decrease. A DVP under 2 cm, or an AFI under 5 cm (or an AFI below the 5th percentile for gestational age); by true volume, under 200 to 500 mL.^[2][11]
• Borderline fluid — an AFI of 5 to 8 cm (or 5 to 10 cm), a grey zone worth knowing because it predicts malformation if found at 24 to 34 weeks.^[2]

^{[1] [2] [5]}

The number on the scan is only as good as the method. Two techniques dominate, and which one you use changes who you label abnormal — so settle the method before you settle the diagnosis.^[3][4]

References12ShowHide

1. [1]Dashe JS, Pressman EK, Hibbard JU SMFM Consult Series #46: Evaluation and management of polyhydramnios Am J Obstet Gynecol, 2018.PMID 30048635
2. [2]Dubil EA, Magann EF Amniotic fluid as a vital sign for fetal wellbeing Australas J Ultrasound Med, 2013.PMID 28191176
3. [3]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.PMID 18646160
4. [4]Moses J, Doherty DA, Magann EF, Chauhan SP, Morrison JC A randomized clinical trial of the intrapartum assessment of amniotic fluid volume: amniotic fluid index versus the single deepest pocket technique Am J Obstet Gynecol, 2004.PMID 15284736
5. [5]Qureshey EJ, Bicocca MJ, Chauhan SP, Nowlen C, Soto EE, Sibai BM, Stafford I Moderate-to-severe polyhydramnios: cutoffs for deepest vertical pocket corresponding to amniotic fluid index J Ultrasound Med, 2022.PMID 35225371
6. [6]van Kempen LEM, van Teeffelen AS, de Ruigh AA, et al. Amnioinfusion compared with no intervention in women with second-trimester rupture of membranes: a randomized controlled trial Obstet Gynecol, 2019.PMID 30531572
7. [7]Van Teeffelen S, Pajkrt E, Willekes C, Van Kuijk SM, Mol BW Transabdominal amnioinfusion for improving fetal outcomes after oligohydramnios secondary to preterm prelabour rupture of membranes before 26 weeks Cochrane Database Syst Rev, 2013.PMID 23913522
8. [8]American College of Obstetricians and Gynecologists ACOG Committee Opinion Number 828: Indications for outpatient antenatal fetal surveillance Obstet Gynecol, 2021.PMID 34011892
9. [9]Martins JG, Biggio JR, Abuhamad A SMFM Consult Series #52: Diagnosis and management of fetal growth restriction 2020.PMID 32407785
10. [10]Hughes DS, Magann EF Antenatal fetal surveillance "Assessment of the AFV". Best Pract Res Clin Obstet Gynaecol, 2017.PMID 27756534
11. [11]Sherer DM A review of amniotic fluid dynamics and the enigma of isolated oligohydramnios Am J Perinatol, 2002.PMID 12152144
12. [12]Magann EF, Chauhan SP, Doherty DA, Lutgendorf MA, Morrison JC A review of idiopathic hydramnios and pregnancy outcomes Obstet Gynecol Surv, 2007.PMID 18005456