Amniotic Fluid Index Calculator

Amniotic fluid at term is largely fetal urine, produced at roughly 800 to 1,200 mL per day, supplemented by lung liquid and cleared by fetal swallowing and intramembranous absorption across the placental surface. Volume therefore reports on renal perfusion and on the patency of the swallowing and gastrointestinal tract, which is why it became a surveillance tool. The index is obtained by dividing the uterus into four quadrants at the umbilicus and linea nigra, measuring the deepest vertical pocket of fluid free of cord and fetal parts in each with the transducer held perpendicular to the floor, and summing the four values in centimeters. Values of 5 to 24 cm are conventionally normal, below 5 cm is oligohydramnios, and above 24 cm is polyhydramnios, graded mild to 30, moderate to 35, and severe beyond that. Total volume rises to about 800 to 1,000 mL near 34 to 36 weeks and then declines, so a value that is unremarkable at 34 weeks may sit below the fifth percentile at 40 weeks; gestational-age-specific reference curves matter more than the fixed 5 cm cutoff. The index is semi-quantitative and correlates imperfectly with dye-dilution measurement of true volume, and inter-observer differences of one to two centimeters can flip a diagnosis near the threshold. Maternal hydration transiently raises the index by three to four centimeters. Randomized comparisons show the four-quadrant method diagnoses more oligohydramnios and triggers more inductions than the single deepest pocket without improving perinatal outcome.

Ultrasound assessment of amniotic fluid began as a subjective impression of whether the fluid looked adequate. The first widely adopted objective rules came from Frank Manning and Lawrence Platt, who built a fluid criterion into the biophysical profile they introduced in 1980, and from Peter Chamberlain and colleagues in Winnipeg, whose 1984 study in the American Journal of Obstetrics and Gynecology graded pregnancies by the largest single vertical pocket and linked pockets under one centimeter to sharply higher perinatal mortality. Jeffrey Phelan and coworkers proposed the four-quadrant sum in 1987 in the Journal of Reproductive Medicine, reasoning that sampling the whole uterine cavity would be less sensitive to where a single pocket happened to lie. Thomas Moore and Jonathan Cayle published gestational-age-specific normal curves for the index in 1990, supplying the percentile framework still in use. Everett Magann and others documented in 2000 that neither method estimates dye-determined volume well, and a Cochrane review comparing the two concluded that the single deepest pocket produced fewer diagnoses of oligohydramnios and fewer inductions with no excess of adverse neonatal outcomes. ACOG practice bulletins on antepartum fetal surveillance now accept either technique.