Defining Oligohydramnios

This year the American College of Obstetricians and Gynecologists (ACOG) restated their support for the use of the deepest vertical pocket (DVP)  of amniotic fluid volume of 2 cm or less to diagnose oligohydramnios (too little amniotic fluid) rather than an amniotic fluid index (AFI) of 5 cm or less [1,2]. The deepest vertical pocket (a.k.a. maximum vertical pocket) method for amniotic fluid assessment is preferred because  clinical trials have shown that defining oligohydramnios as a deepest vertical pocket of less than 2 cm or less will result in fewer obstetric interventions without a change  in adverse outcomes when compared with defining oligohydramnios as an AFI of less than or equal to 5 cm [3].

Deepest Vertical Pocket of Amniotic Fluid In addition , the DVP is a simpler method to use than the AFI method. The DVP is measured as the vertical measurement , in centimeters, of the single deepest pocket of amniotic fluid with a transverse measurement of 1 cm or more wide  without fetal small parts or umbilical cord  [4] . The AFI method uses the sum of measurements obtained for the deepest vertical pocket from four quadrants of the uterus, using the same criteria otherwise.

The DVP is already used more commonly in the evaluation of twin pregnancies [5-7] . However, the AFI is used by many antenatal testing centers as part of the the modified biophysical profile (BPP) [8,9]. The modified BPP consists of the nonstress test (NST) and an amniotic fluid volume assessment using the AFI method. The modified BPP is considered normal if both the NST is reactive and the AFI is greater than 5 cm [1]. The new recommendations would require only a single maximum vertical pocket of amniotic fluid greater than 2 cm for the amniotic fluid volume assessment component of the modified BPP to  be considered normal .

The  normal the range for the deepest vertical pocket is about 2 cm  to 8 cm in singleton and twin gestations [4, 10-12] , or about one third the values for  the normal range of the AFI.  

Depth of largest visible
pocket 

 Qualitative Description

 < 1 cm
 severe oligohydramnios
> 1 and
< 2  cm
 mild oligohydramnios
 > 2  and  < 8 cm  normal
> 8
and < 12 cm
 polyhydramnios
>12 cm and <
16
 moderate polyhydramnios
> 16 cm  severe polyhydramnios

Some causes of oligohydramnios [13,14]

  • Ruptured membranes
  • Congenital abnormalities
    • Bilateral renal agenesis or cystic dyplasia
    • Obstruction of the urinary tract
    • Meckel-Gruber syndrome
    • VACTERL (vertebral, anal, cardiac, tracheo-esophageal, renal, limb) association
    • Sirenomelia
    • Sacral agenesis
  • Growth restriction (placental insufficiency)
  • Postterm pregnancy
  • Drugs
    • Angiotensin-converting enzyme inhibitors
    • Prostaglandin synthase inhibitors
  • Twin- to -twin transfusion
  • TRAP (twin reverse arterial perfusion sequence)
  • Fetal demise
  • Idiopathic

REFERENCES
1. Antepartum fetal surveillance. Practice Bulletin No. 145. American College of Obstetricians and Gynecologists. Obstet Gynecol 2014;124:182–92PMID:24945455
2. Reddy UM, et al.Fetal Imaging: Executive Summary of a Joint Eunice Kennedy Shriver National Institute of Child Health and Human Development, Society for Maternal-Fetal Medicine, American Institute of Ultrasound in Medicine, American College of Obstetricians and Gynecologists, American College of Radiology, Society for Pediatric Radiology, and Society of Radiologists in Ultrasound Fetal Imaging Workshop Obstet Gynecol. 2014 May;123(5):1070-82. doi: 10.1097/AOG.0000000000000245. PMID: 24785860
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 Jul 16;(3):CD006593. doi: 10.1002/14651858.CD006593.pub2. PMID: 18646160
4. Chamberlain PF, Manning FA, Morrison I, Harman CR, Lange IR. Ultrasound evaluation of amniotic fluid volume. I. The relationship of marginal and decreased amniotic fluid volume to perinatal outcome. Am J Obstet Gynecol 1984;150:245–9.PMID:6385713
5.Magann EF, et al. The ultrasound estimation of amniotic fluid volume in diamniotic twin pregnancies and prediction of peripartum outcomes. Am J Obstet Gynecol. 2007 Jun;196(6):570.e1-6; discussion 570.e6-8. PMID: 17547899
6.Royal College of Obstetricians and Gynaecologists .Monochorionic Twin Pregnancy, Management Green-top Guideline No. 51December 2008
https://www.rcog.org.uk/globalassets/documents/guidelines/t51managementmonochorionictwinpregnancy2008a.pdf
7. WAPM Consensus Group on Twin-to-Twin Transfusion, Baschat A, et. al. Twin-to-twin transfusion syndrome (TTTS). J Perinat Med. 2011 Mar;39(2):107-12. Epub 2010 Dec 13. PMID:21142846
8. Nageotte MP, et. al. Perinatal outcome with the modified biophysical profile. Am J Obstet Gynecol. 1994 Jun;170(6):1672-6. PMID:8203424
9.Miller DA, et. al., The modified biophysical profile: antepartum testing in the 1990s.Am J Obstet Gynecol. 1996 Mar;174(3):812-7. PMID:863364
10.Chamberlain PF, Ultrasound evaluation of amniotic fluid volume. II. The relationship of increased amniotic fluid volume to perinatal outcome. Am J Obstet Gynecol. 1984 Oct 1;150(3):250-4.PMID:6385714
11.Hill LM, et al. Polyhydramnios: ultrasonically detected prevalence and neonatal outcome.Obstet Gynecol. 1987 Jan;69(1):21-5.PMID:3540761
12. Magann EF,et al The ultrasound estimation of amniotic fluid volume in diamniotic twin pregnancies and prediction of peripartum outcomes. Am J Obstet Gynecol. 2007;196(6):570.PMID:17547899
13. Peipert JF, Donnenfeld AE, Oligohydramnios: a review.Obstet Gynecol Surv. 1991 Jun;46(6):325-39. PMID:2067755
14. McCurdy CM Jr, Seeds JW. Oligohydramnios: problems and treatment. Semin Perinatol. 1993 Jun;17(3):183-96. PMID:7690990

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