Early transvaginal biometry of fetal orbits: a cross-sectional study.

OBJECTIVE: To construct nomograms of orbital diameters in early pregnancy by transvaginal sonography. DESIGN: A prospective cross-sectional study. METHODS: The study was performed on 923 consecutive normal singleton pregnancies from 10 to 16 weeks of gestation, using transvaginal high-resolution ultrasound technique. The interocular distance (IOD), binocular distance (BOD), as well as the orbital diameter (OD) at each gestational age (GA, days), were recorded. The distribution of the residuals for the different orbital measurements are also described. RESULTS: The increase in each of the ocular parameters studied could be accurately described by a linear model. The regression equation for orbital measurements as a function of GA was: for IOD, y = 0.153 GA (days) - 6.73 (r2 = 0.82); for BOD, y = 0.387 GA (days) - 16.85 (r2 = 0.80), and for OD, y = 0.132 GA (days) - 6.435 (r2 = 0.79). IOD and OD demonstrate a normal distribution of the residuals with uniform variance. CONCLUSION: The presented data obtained in early pregnancy by transvaginal scan offer normative measurements of the orbital diameter which may be useful in the prenatal diagnosis of congenital syndromes that include, among other manifestations, orbit maldevelopment and growth disturbances.