Prior knowledge of obstetric gestational age and possible bias of Ballard score

Objective: To determine whether prior knowledge of obstetric estimate of ge stational age creates a bias in assignment of gestational age by the Ballar d assessment score. Methods: The Ballard assessment score,vas done on 82 infants weighing less than 2500 g by two examiners, one who had prior knowledge of best obstetric gestational age estimate and the other who was masked to that information. Obstetric gestational age was correlated with masked and unmasked score. S tatistical analysis used Spearman rank correlation test, plotting the measu rement means against the measurement differences, chi(2), Wilcoxon signed-r ank test, analysis of variance, and Student t test. Significance was assume d at P < .05. Results: There was high reliability (r = 0.84-0.86) between obstetric gesta tional age estimates (33.3 +/- 3.0 weeks) and the gestational age derived f rom masked (34.3 +/- 3.0 weeks) and unmasked (34.0 +/- 3.0 weeks) Ballard s cores. The mean difference between unmasked scores and gestational age was 1.38 +/- 1.15 weeks and that between masked scores and obstetric gestationa l age was 1.40 +/- 1.15 weeks, a nonsignificant difference. There was no si gnificant difference in the number of cases with 100% agreement between mas ked and unmasked scores and obstetric gestational age or in the number of c ases in which the score (masked or unmasked) differed by more than 2 weeks from obstetric gestational age. This study had over 99% power to detect a I -week difference between the accuracy of masked and unmasked scores. Conclusion: Prior knowledge of obstetric gestational age did not bias the B allard assessment score. (C) 1999 by The American College of Obstetricians and Gynecologists.