Ultrasonic planimetry of the gestation sac as a biometric method in early pregnancy.

A new method for determining the area of the gestation sac by thread planimetry is described. Mean weekly values with two standard deviations were determined on a series of 237 measurements of gestation sac area, age five to 11 weeks, in normal pregnancies. The gestation sac area grows curvilinearly from 1.85 cm2 at five weeks to 15 cm2 at 11 weeks. The mean diameter was calculated from the gestation sac area by using an adequate approximation of the irregular gestation sac shape to a circle of the same area. The dependence of the mean gestation sac diameter on gestation age is defined by the regression equation y = 0.46x - 0.95 and the correlation factor r = 0.92. Practical applications of gestation sac planimetry include the determination of gestation age in early pregnancy, and the follow-up of growth or determination of delayed growth in disturbed or failed early pregnancy.