One, two or three: how many directions are useful in transvaginal ultrasound measurement of ovarian follicles?

Objective: To investigate whether it is necessary to measure an ovarian follicle in three directions using transvaginal ultrasonography. Methods: In 36 healthy female volunteers transvaginal ultrasonography was performed every other day during a spontaneous menstrual cycle. The diameter of the largest follicle in each ovary was measured in two directions in the sagittal plane, and in two directions in the coronal plane. In total, 304 follicular measurements were performed. The largest follicular diameter was compared to the mean diameter of two and three directions, respectively. The mean diameter of two directions was compared to that of three directions. Results: The mean difference between measurement in one and two directions was 1.2 mm (standard deviation (S.D.) = 1.1 mm), between measurement in one and three directions 1.2 mm (S.D.=1.0 mm), and between measurement in two and three directions −0.03 mm (S.D.=0.3 mm). The mean difference and the standard deviation of the difference increased with the follicular diameter. Conclusions: In non-stimulated menstrual cycles, follicle measurement in only one direction is less accurate than measurement in two and three directions, and may result in clinically relevant differences. However, measurement in three directions gives no additional information compared to measurement in two directions.