Conditional centiles for the quantification of fetal growth.

A total of 274 low-risk pregnancies underwent predetermined, scheduled, serial ultrasound examinations and measurement of the biparietal diameter, fetal abdominal area, femur length and estimated fetal weight. The longitudinal nature of this dataset permits the construction of conditional centiles for these measurements, whereby the dimensions of an individual fetus at a given gestational age can be extrapolated to give a range of expected values (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 quantification and subsequent evaluation of time-dependent changes in fetal dimensions to be made.