Misclassification of newborns due to systematic error in plotting birth weight percentile values.

OBJECTIVES Higher than expected small for gestational age (SGA) rates and lower than expected large for gestational age (LGA) rates have been observed. A possible explanation is a leftward shift of percentile curves for birth weight due to a systematic error in plotting birth weight values in charts (ie, plotting weekly mean birth weight data at the beginning of the weeks). Our objectives were to assess how common this plotting error is and to analyze the effect of this error on SGA and LGA classification based on data from the German perinatal survey. METHODS First, a systematic literature search for birth weight charts was performed, and the charts were analyzed for the plotting error. Second, percentile values (10th, 50th, and 90th) for 25 to 42 completed weeks of gestation were calculated from the data of 1181200 male singleton newborns (German perinatal survey, 1995-2000). Birth weight percentile curves were calculated with and without the plotting error, and the resulting SGA and LGA rates were analyzed. RESULTS Fourteen of the 16 identified publications contained the systematic error in plotting. Using our calculated percentile curves, a leftward shift caused by the plotting error led to an SGA rate of 12.5% and an LGA rate of 7.7%; ∼5% of newborns were misclassified. CONCLUSIONS Percentile charts should be examined for the described systematic error and, if necessary, corrected.