ObjectiveTo determine whether prior knowledge of obstetric estimate of gestational age creates a bias in assignment of gestational age by the Ballard assessment score.MethodsThe Ballard assessment score was 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. Statistical analysis used Spearman rank correlation test, plotting the measurement means against the measurement differences, χ2, Wilcoxon signed-rank test, analysis of variance, and Studentttest. Significance was assumed atP< .05.ResultsThere was high reliability (r= 0.84–0.86) between obstetric gestational age estimates (33.3 ± 3.0 weeks) and the gestational age derived from masked (34.3 ± 3.0 weeks) and unmasked (34.0 ± 3.0 weeks) Ballard scores. The mean difference between unmasked scores and gestational age was 1.38 ± 1.15 weeks and that between masked scores and obstetric gestational age was 1.40 ± 1.15 weeks, a nonsignificant difference. There was no significant difference in the number of cases with 100% agreement between masked and unmasked scores and obstetric gestational age or in the number of cases 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 1-week difference between the accuracy of masked and unmasked scores.ConclusionPrior knowledge of obstetric gestational age did not bias the Ballard assessment score.