Amniotic fluid index measurements in normal pregnancy after 28 gestational weeks.

OBJECTIVE The purpose of this study is to establish a normative scale of amniotic fluid index (AFI) or four-quadrant amniotic fluid index throughout gestation in uncomplicated singleton pregnancies, and to identify the lower and upper limits for each gestational week. METHOD A prospective longitudinal study was used. One hundred seventeen uncomplicated singleton pregnancies were examined every 4 weeks between 28 and 42 week's gestation. The uterine cavity was divided into four quadrants. With the use of linear-array, real-time B-scanning, the vertical diameter of the largest pocket in each quadrant was measured. The amniotic fluid index is the sum of these four quadrants. RESULTS The amniotic fluid index observations from regression equation curve were stratified in week-specific normative curves. The variation between mean AFI of the total population and the means of the preterm was significantly greater than term pregnancies (P<. 05). The AFI 2.5%, 5%, 10%, 90% 95% and 97.5% limits about the 50th (124 mm) were 68, 81, 90, 135, 144 and 145 mm, respectively, in term gestation. The 5th and 95th percentile serves as lower and upper limits of normal, respectively for 28-42 weeks gestation. CONCLUSIONS Gestational age-specific values of AFI were established, determining the significant trends of changes in the amniotic fluid volume with gestation. The normogram may have a clinical benefit to accurate, reliable and semiquantitative diagnosis of oligohydramnios and polyhydramnios.