Background <p>Post-spinal anesthesia hypotension during cesarean delivery results from reduced systemic vascular resistance due to the blockade of autonomic nerves, which is further worsened by inferior vena cava (IVC) compression by the gravid uterus. The study aimed to determine whether venous flow per respiratory cycle of right external iliac vein flow (REIVF) in the inguinal region, as measured on ultrasound in the deferent positions, could reflect the degree of IVC compression and further identify parturients at risk of post-spinal hypotension.</p> Methods <p>One hundred and twenty parturients scheduled for elective cesarean delivery under spinal anesthesia were enrolled in this study. Venous flow (VF) per respiratory cycle of the REIVF was measured using ultrasound before anesthesia in the supine position and a left tilt of 15°(A 15° slope pad is placed under the waist of the pregnant woman)Respond 1. All patients were divided into a hypotension group (<i>n</i> = 64) and a non-hypotension group (<i>n</i> = 53) Respond 11. based on whether SHS occurred. The primary outcomes included preoperative ultrasound parameters of REIVF obtained in different positions, the correlation between these parameters and SHS, and the magnitude of changes in REIVF induced by postural alteration (ΔREIVF). Hypotension was defined as a more than 20% reduction in systolic blood pressure relative to baseline. Respond 4. Multivariate logistic regression analysis was performed to explore the relationship between ΔREIVF and post-spinal hypotension. Receiver operating characteristic (ROC) curves were constructed to evaluate the predictive performance of relevant parameters for post-spinal hypotension. The corresponding area under the curve (AUC) and optimal cut-off values were subsequently calculated.</p> Results <p>At a 15° left tilt, REIVF was significantly higher in the non-hypotensive group than in the hypotensive group. In both groups, REIVF per respiratory cycle at a 15° left tilt was significantly higher than that in the supine position (<i>P</i> &lt; 0.001).The difference in right external iliac vein flow between the supine position and 15° left tilt (ΔREIVF) was independently associated with post-spinal hypotension (SHS) following spinal anesthesia for cesarean delivery (OR = 1.071, 95% CI: 1.038–1.104, <i>P</i> &lt; 0.001).Although systolic blood pressure (SBP) declined in both subgroups (ΔREIVF &gt; 125.85 mL/min vs. ΔREIVF ≤ 125.85 mL/min), the reduction in SBP was markedly greater in the ΔREIVF &gt; 125.85 mL/min group at 4 and 5&#xa0;min after spinal anesthesia (<i>P</i> = 0.001 and <i>P</i> &lt; 0.001, respectively). In addition, heart rate (HR) increased significantly in the ΔREIVF ≤ 125.85 mL/min group at 4&#xa0;min after spinal anesthesia (<i>P</i> = 0.011).</p> Conclusion <p>Variations in REIVF among different positions prior to anesthesia are predictive of subsequent post-spinal hypotension (SHS). Respond 3.</p> Trial registration <p>This study was registered at <a href="http://www.chictr.org.cn">http://www.chictr.org.cn</a>, No. ChiCTR2100046925, 2021-05-31.</p>