Amniotic Fluid and the Clinical Relevance of the Sonographically Estimated Amniotic Fluid Volume

Amniotic Fluid and the Clinical Relevance of the Sonographically Estimated Amniotic Fluid Volume Oligohydramnios Dynamics of the Amniotic Fluid Volume Amniotic fluid provides an ideal environment for normal fetal growth and development. It provides the fetus with a source of water, protects the fetus from trauma, allows for normal movements critical for anatomic development, and contributes to the development of the fetal lungs. 1 The physiologic characteristics of the dynamics influencing the amniotic fluid volume (AFV) are complex and not clearly understood. To understand the AFV, knowledge of the pathways for potential amniotic fluid movement and the regu- latory mechanisms involved must be taken into consideration. There are several potential sources influencing AFV: fetal urine production, fetal swallowing, secretion of fetal lung fluid, the in- tramembranous pathway (movement of water and solutes between amniotic fluid and fetal blood and the placenta), the transmembra- Everett F. Magann, MD, Adam T. Sandlin, MD, Songthip T. Ounpraseuth, PhD Received February 22, 2011, from the Depart- ments of Obstetrics and Gynecology (E.F.M., A.T.S.) and Biostatistics (S.T.O.), University of Arkansas for the Medical Sciences, Little Rock, Arkansas USA. Revision requested March 30, 2011. Revised manuscript accepted for publication May 22, 2011. Address correspondence to Everett F. Magann, MD, Department of Obstetrics and Gynecology, University of Arkansas for the Medical Sciences, 4301 W Markham St, Slot 518, Little Rock, AR 72205 USA. E-mail: [email protected] Abbreviations AFI, amniotic fluid index; AFV, amniotic fluid volume; NICU, neonatal intensive care unit ©2011 by the American Institute of Ultrasound in Medicine | J Ultrasound Med 2011; 30:1573–1585 | 0278-4297 | www.aium.org STATE-OF-THE-ART REVIEW Invited paper The amniotic fluid volume (AFV) is regulated by several systems, including the in- tramembranous pathway, fetal production (fetal urine and lung fluid) and uptake (fetal swallowing), and the balance of fluid movement via osmotic gradients. The normal AFV across gestation has not been clearly defined; consequently, abnormal volumes are also poorly defined. Actual AFVs can be measured by dye dilution techniques and directly measured at cesarean delivery; however, these techniques are time-consuming, are invasive, and require laboratory support, and direct measurement can only be done at cesarean delivery. As a result of these limitations, the AFV is estimated by the amniotic fluid index (AFI), the single deepest pocket, and subjective assessment of the AFV. Unfortunately, sonographic estimates of the AFV correlate poorly with dye-determined or directly measured amniotic fluid. The recent use of color Doppler sonography has not improved the diagnostic accuracy of sonographic estimates of the AFV but instead has led to overdiagnosis of oligohydramnios. The relationship between the fixed cutoffs of an AFI of 5 cm or less and a single deepest pocket of 2 cm or less for identifying adverse pregnancy outcomes is uncertain. The use of the single deepest pocket compared to the AFI to identify oligohydramnios in at-risk pregnancies seems to be a better choice because the use of the AFI leads to an increase in the diagnosis of oligohydramnios, re- sulting in more labor inductions and cesarean deliveries without any improvement in peripartum outcomes. Key Words—amniotic fluid; amniotic fluid index; oligohydramnios; single deepest pocket

Amniotic Fluid and the Clinical Relevance of the Sonographically Estimated Amniotic Fluid Volume

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