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PURPOSE: To determine the values of amniotic fluid in normal fetuses during the first trimester of pregnancy by three- and bi-dimensional ultrasonography. METHODS: In a prospective longitudinal study, 25 normal fetuses were evaluated from the 8th to the 11th week of gestation. Amniotic fluid volume was measured by endovaginal ultrasonography with the three- and two-dimensional modes. The two-dimensional study consisted of volumetric determination by mathematical calculation based on an ellipsoidal shape (constant 0.52) to obtain the amniotic sac and embryo volumes. In the three-dimensional study, the amniotic fluid volume was determined by the VOCAL technique using 6, 9, 15, and 30 degrees of rotation. The amniotic fluid volume obtained by 6-degree rotations was considered to be the final result. In both modes, amniotic fluid volume was obtained by subtracting the volume of the embryo from the volume of the amniotic sac. Data were analyzed statistically for variance (ANOVA), correlation and regression analysis. The level of significance was set at p < 0.05. RESULTS: The amniotic fluid volume as measured by two-dimensional ultrasonography increased from 5.45 to 39.52 cm³ in the range from the 8th to the 11th week (ANOVA - p < 0.05). There was a correlation between gestational age and amniotic fluid volume (p < 0.001, r² = 88.3%). In the three-dimensional study, the amniotic fluid volume increased from 5.7 to 42.9 cm³ in the range from the 8th to the 11th week (ANOVA - p < 0.05), and again a correlation between gestational age and amniotic fluid volume (p < 0.001, r² = 98.1%) was observed. CONCLUSION: an increase in amniotic fluid volume occurs during the first trimester of pregnancy, as determined by the two- and three-dimensional modes. In addition, we have demonstrated that the higher the gestational age, the larger the amniotic fluid volume.