Ultrasound Evaluation of Two Anatomic Landmarks in Spinal Anesthesia for Cesarean Section: the Posterior Superior Iliac Spine VS Iliac Crest

The investigators performed a two-stage study to evaluated the accuracy of Intercristal line and the PSIS line to identify the lumbar intervertebral space for spinal anesthesia using ultrasonography in pregnant woman. In phase 1, the distances between the PSIS line and each lumbar interspinous space is measured by ultrasound,and the mean and 95% confidence interval of distances in pregnant women with different height ranges were calculated to provide a reference for identifying specific lumbar spaces.In phase 2, a further 230 participants were received the PSIS, Intercristal line or both as palpation landmarks to define specific lumbar spaces.