Abstract 1324 Poster Session IV, Tuesday, 5/4 (poster 260)

Objective: Standardized scoring systems such as the Dubowitz and Ballard examinations are used to estimate gestational age at birth. These are especially important prognosticators in premature or low birth weight infants in whom obstetric gestational age is unknown (such as women who receive limited prenatal care or are uncertain about their last menstrual period). Prior knowledge of obstetric gestational age may bias performance of the Ballard. This would artificially lead to greater concordance with "known" obstetric gestational age. The objective of this study was to determine if prior knowledge of obstetric estimate of gestational age creates a bias in assignment of gestational age by the Ballard assessment score (BAS).

Methods: The BAS was performed on 82 infants weighing <2500 g by two examiners, one who had prior knowledge of best estimate of obstetric gestational age and the other masked to this information. Correlation between obstetric gestational age and masked / unmasked BAS was performed. Statistical analysis was performed using Spearman rank correlation, plotting the measurement means against the measurement differences, Chi Square, Wilcoxon signed rank, analysis of variance and Student's t tests. Significance was assumed at p<0.05.

Results: A priori bias between examiners was excluded using the rank correlation test (r = 0.85). There was high reliability (r = 0.84 - 0.86) between obstetric gestational age (33.3 ± 3.0 weeks) and gestational age derived from masked (34.3 ± 3.0 weeks) and unmasked (34.0 ± 3.0 weeks) BAS. The mean difference between unmasked BAS and obstetric gestational age was 1.38 ± 1.15 weeks and the mean difference between masked BAS and obstetric gestational age was 1.40 ± 1.15 weeks, a non-significant difference. There was no significant difference in the number of cases with 100% agreement between masked or unmasked BAS and obstetric gestational age (23.8% vs 26.8%, respectively) or in the number of cases in which BAS (masked or unmasked) differed by more than 2 weeks from obstetric gestational age (15% vs 12.5%, respectively). This study had 99% power to detect a 1 week difference between the accuracy of masked and unmasked BAS.

Conclusion: This is the first study to prospectively assess potential bias of prior knowledge of obstetric gestational age in assignment of the Ballard score. We conclude that prior knowledge of obstetric gestational age does not bias the Ballard assessment score.