We evaluated normal uterine involution prospectively with real-time ultrasonography in 100 women after uncomplicated term vaginal delivery. Transducers easily distorted the spongy uterus during early postpartum scanning, an effect minimized with sector transducers that are superior to linear or convex probes for accurate early postpartum uterine measurement. Long-axis measurements correcting for uterine angulation were the most reproducible and accurate, irrespective of bladder distention. Uterine contractions caused instability of particularly the transverse dimension. Increasing maternal parity was associated with slightly but significantly larger uterine dimensions up to 4 weeks post partum. The uterus reassumed nongravid dimensions by 6 to 8 postpartum weeks. Uterine involution was unrelated to infant birth weight or breast feeding.