Fetal binocular distance as a predictor of menstrual age.

The relation between fetal binocular distance and menstrual age was determined by cross-sectional analysis of 555 normal fetuses (14-40 weeks) using real-time sonography. Mathematical modelling of the data demonstrated that the binocular distance growth curve, similar to the biparietal diameter, is nonlinear. Predicted binocular values at various points in gestation were comparable to the results of other investigators. Predicted menstrual age in weeks for specific binocular distance measurements in millimeters were calculated and are reported in tabular form. The variability (+/- 2 SD) associated with predicting menstrual age from binocular distance is +/- 14 days between 14 and 27 weeks, but between 29 and 40 weeks the variability is +/- 24 days. Binocular distance can be used as an adjunct in estimating menstrual age and may be useful in the diagnosis of some abnormalities, e.g. hypotelorism or hypertelorism.