The life style, genetic predisposition and metabolic changes occurring during pregnancy can modify the percent value of glycated hemoglobins (HbA1 and HbA1c). In addition, research papers from different laboratories in the world have reported contradictory results on this respect. The purpose of this trial was to know the percent value of HbA1 in healthy women, during the different trimesters of pregnancy. 206 pregnant (E) healthy women who came over for prenatal control to UMF No 80 IMSS in Morelia, Michoacan with no previous history of Diabetes mellitus or Essential Hypertension were classified by trimesters of pregnancy (1T, 2T, 3T) and chronological age (I, 18-24; 11, 25-30; III, 31-35 years). Their chronological and gestational ages, weight, height, body mass index and parity were recorded. % HbA1 (ion exchange chromatography) was determined on each patient. Control group was formed by 187 non pregnant healthy women (NE) chosen with same criterion that pregnant women. % HbA1 was lower in E during pregnancy (7.11 +/- 1.53 vs 7.78 +/- 1.12%, p < 0.0001) than NE group. % HbA1 in E group was lower in the 1T and 2T than in the 3T (p < 0.001), same situation was observed in 18 to 24 (group I) and 25 to 30 (group II) years old. In the other hand, in E from group II on the 2T the weeks of gestation were correlated with % HbA1 (r = 0.72, p < 0.05). This results show a diminished HbA1 percent in E group with a lower values in the 1T and 2T. Moreover, these results will allow us to know HbA1 appearance in diabetic pregnant women and to evaluate the degree of metabolic control.