Standards for the quantification of serial changes in the amniotic fluid index.

A total of 274 low-risk pregnancies underwent predetermined scheduled serial ultrasound examinations and measurements of the amniotic fluid index (AFI). As expected from previous cross-sectional studies, there is a gradual decline in the AFI from mid-trimester until term. The longitudinal nature of this data set permits the construction of conditional centiles for AFI whereby the AFI of an individual fetus at a given gestational age can be extrapolated to give a range of expected values for AFI (expressed as centiles) at a later gestational age. Examples of conditional centiles based upon 4-week measurement separations are presented. These conditional centiles permit the appropriate evaluation and quantification of time-dependent changes in AFI to be made.