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Amniotic Fluid (AF)

Amniotic fluid is the fluid that surrounds the fetus. Early in gestation amniotic fluid composition resembles protein-free fetal serum, [1] a result of fluid transudation across the fetal skin or the maternal decidua. After approximately 11 weeks' gestation amniotic fluid is nearly all fetal urine with a small amount of fluid contributed by the lungs.

Amniotic fluid volume appears to be determined by a balance between the inflows from fetal urine, and outflows of fetal swallowing and intramembranous water flow (from amniotic fluid across the amnion into the fetal-placental vasculature)  [2,3]. The amniotic fluid accumulates throughout the first two trimesters and reaches a peak volume  at 33 to 34 weeks [4].

Some causes of polyhydramnios (too much amniotic fluid) [5]

  • Idiopathic (unknown) ~ most cases
  • Gastrointestinal abnormalities
    • Esophageal atresia
    • Intestinal obstruction
    Central nervous system abnormalities
    • Anencephaly
  • Chromosomal abnormalities
  • Nonimmune hydrops
  • Skeletal dysplasias
  • Diabetes
  • Twin- to -twin transfusion
Some causes of  oligohydramnios (too little amniotic fluid) [6,7]

  • 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

Ultrasound may be used to obtain a qualitative measure of the amount of amniotic fluid present. One ultrasound method of assessing the AF is to measure  the depth of the largest visible  pocket of fluid surrounding the fetus. The normal range for the deepest vertical pocket (or maximum vertical pocket) is 2 cm  to 8 cm in singleton gestations [8-10]. The normal range for the single deepest pocket in twin gestation appears to be 2.2 cm to 7.5 cm [11]
 

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
 

Another method for assessing the AF, called the Amniotic Fluid Index (AFI), uses the sum of the deepest vertical pocket of fluid found in each quadrant of the uterus the normal range for the AFI is 5 to 25 cm [12,13]. Both methods are qualitative measures of the AF and  may be a poor reflection of the actual amniotic fluid volume [14].

REFERENCES:

1. Lind T. The biochemistry of amniotic fluid. In: Fairweather DVI, Eskes TKAB, editors. Amniotic fluid research and clinical application. Amsterdam: Excerpta Medica; 1973. p. 60-81.
2. Beall MH, et al.,Regulation of amniotic fluid volume. Placenta. 2007 Aug-Sep;28(8-9):824-32. Epub 2007 Feb 15.PMID:17303237
3. Robertson P, Responses of amniotic fluid volume and its four major flows to lung liquid diversion and amniotic infusion in the ovine fetus. Reprod Sci. 2009 Jan;16(1):88-93. PMID:19144891
4.Brace RA, Wolf EJ.Normal amniotic fluid volume changes throughout pregnancy. Am J Obstet Gynecol. 1989 Aug;161(2):382-8. PMID:2764058
5.Damato N, et al. Frequency of fetal anomalies in sonographically detected polyhydramnios. J Ultrasound Med. 1993 Jan;12(1):11-5. PMID:8455215
6. Peipert JF, Donnenfeld AE, Oligohydramnios: a review.Obstet Gynecol Surv. 1991 Jun;46(6):325-39. PMID:2067755
7. McCurdy CM Jr, Seeds JW. Oligohydramnios: problems and treatment. Semin Perinatol. 1993 Jun;17(3):183-96. PMID:7690990
8. Chamberlain PF,Ultrasound evaluation of amniotic fluid volume. I. The relationship of marginal and decreased amniotic fluid volumes to perinatal outcome. Am J Obstet Gynecol. 1984 Oct 1;150(3):245-9. PMID: 6385713
9.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
10.Hill LM, et al. Polyhydramnios: ultrasonically detected prevalence and neonatal outcome.Obstet Gynecol. 1987 Jan;69(1):21-5. PMID:3540761
11. 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
12. Rutherford SE, Phelan JP, Smith CV, Jacobs N.The four-quadrant assessment of amniotic fluid volume: an adjunct to antepartum fetal heart rate testing. Obstet Gynecol. 1987 Sep;70(3 Pt 1):353-6. PMID: 3306497
13. Phelan JP, Smith CV, Broussard P, Small M. Amniotic fluid volume assessment with the four-quadrant technique at 36-42 weeks' gestation. J Reprod Med. 1987 Jul;32(7):540-2.PMID: 3305930
14. Chauhan SP,et al. Ultrasonographic assessment of amniotic fluid does not reflect actual amniotic fluid volume. Am J Obstet Gynecol. 1997 Aug;177(2):291-6; discussion 296-7.
PMID: 9290442

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