The study was conducted to construct a normal range for the diameters of the aortic root and pulmonary artery during the first half of gestation. With the use of transvaginal and transabdominal high-resolution ultrasound techniques, a prospective, cross-sectional study was performed on 139 normal singleton pregnancies at between 14 and 26 weeks. Great vessel diameters were measured by transvaginal ultrasonography until 17 weeks, and by transabdominal ultrasound between 18 and 26 weeks' gestation. The results showed that the aortic diameter (AD) as a function of gestational age (GA) was expressed by the regression equation AD = -16.0331 + 2.2563 x GA, and the pulmonary artery diameter (PD) by PD = -14.7637 + 2.4026 x GA; AD and PD are expressed in millimeters and GA in weeks. The correlation r2 = 0.94 was found to be highly statistically significant (p < 0.0001) for both great vessels. The normal mean of aortic and pulmonary artery diameter per week and the 95% prediction limits were also defined. During the study period we evaluated two cases with arterial diameters outside the 95% confidence limits; one had aortic coarctation and the other tetralogy of Fallot. The normative data established by us may be helpful in the prenatal diagnosis of congenital heart defects that include, among their manifestations, discordant diameters of the great vessels.