Magnetic resonance imaging measurements for the dimensions of the gestational sac. Diameter “a” is the maximum longitudinal diameter of the gestational sac, which is measured as the distance between the two ends of the longitudinal axis. Diameter “b” is the maximum transverse diameter of the gestational sac, which is measured as the distance from the most anterior of the gestational sac at the incision to the posterior uterine wall of the lower uterine segment. Thinning or absence of the myometrium between the gestational sac and the bladder can be seen (blue arrow). The dotted yellow line on a sagittal section of the uterus indicates the level of a previous lower uterine segment cesarean section scar.