Sonographic amniotic fluid measurement and fetal growth retardation: a reappraisal.

Sonographic estimation of amniotic fluid volume is commonly used as a diagnostic test for oligohydramnios associated with fetal malnutrition. However, methods of measurement and diagnostic criteria are controversial. We prospectively evaluated the reproducibility of amniotic fluid diameter measurement and studied various threshold definitions of oligohydramnios as predictors of small for gestational age fetuses or fetuses with low ponderal index values. An average of the vertical and two perpendicular horizontal diameters of the largest pocket of amniotic fluid was more reproducible than the maximum vertical pocket and better enabled a single observer to differentiate between patients. An average diameter of 10 mm was highly diagnostic of small for gestational age fetuses but was insufficiently sensitive (27%) to serve as a screening test. Although also of limited sensitivity (40%), an average diameter of approximately 30 mm was probably the best screening threshold. Reduced average diameter was significantly related to the occurrence of a ponderal index less than or equal to the tenth percentile for dates. Three of seven (43%) neonates with a ponderal index less than or equal to the tenth percentile and a birth weight greater than the tenth percentile for dates had a low average diameter, suggesting that reduced average amniotic fluid diameter may help to identify the malnourished fetus with birth weight greater than the tenth percentile for dates.