The purpose of this study was to evaluate in vivo the changes in myometrial thickness throughout pregnancy. Myometrial thickness was measured in 25 singleton uncomplicated pregnancies. Ultrasonographic sagittal and transverse sections were used to measure uterine wall thickness from the low anterior wall (lower segment) and the anterior, posterior, right, and left walls of the upper segment and from the fundus. In each case four measurements were made in the second and third trimesters. Myometrial thickness of the upper uterine segment remains fairly constant in the first and second trimesters of pregnancy, whereas a significant linear trend was found between a decreasing thickness of the lower uterine segment and advancing gestational age. Myometrial thickness is significantly increased behind the placental insertion site as compared to other portions of the uterine wall. These data may serve as baseline reference values for further studies in the antepartum fetal surveillance of high-risk pregnancies.