Trimester-specific reference interval for glycated hemoglobin in non-diabetic pregnant women: A cross-sectional study.

BACKGROUND: Gestational Diabetes Mellitus (GDM) is a growing public health concern in Ethiopia. HbA1c is useful for assessing glycemia but is influenced by physiological changes in pregnancy. To our knowledge, no published trimester-specific HbA1c reference intervals exist for Ethiopian non-diabetic pregnant women. This limits population-specific clinical interpretation. METHODS: A facility-based cross-sectional study was conducted from February to July 2025 among 424 non-diabetic pregnant women across all trimesters. Normality of HbA1c distribution was assessed using the Shapiro-Wilk and Kolmogorov-Smirnov tests; outliers were identified using Tukey's fences method. Reference intervals were determined parametrically (mean ±2SD). Differences across trimesters were tested using one-way ANOVA. Because Levene's test indicated a violation of the homogeneity of variances (p < 0.001) and group sizes were unequal, pairwise comparisons were performed using the Games-Howell post hoc test. A p-value < 0.05 was considered statistically significant. RESULTS: HbA1c showed a normal distribution across all trimesters. Reference intervals were 4.8-5.9% in the first, 4.2-5.8% in the second, and 4.5-5.9% in the third trimester. Mean HbA1c levels differed significantly between trimesters (p < 0.05). Lowest in the second trimester 5.00% (95% CI 4.93-5.07) and highest in the first trimester 5.35% (95% CI: 5.31-5.39). Games-Howell post hoc tests confirmed that all pairwise comparisons were significant (p < 0.05). CONCLUSION: Trimester-specific HbA1c reference intervals were defined for non-diabetic pregnant women, demonstrating measurable physiological variation across gestation. The use of locally derived thresholds could improve the accuracy of GDM screening and support more appropriate clinical interpretation.