In-vivo magnetic resonance imaging (MRI) of the fetus is commonly carried out in the left lateral position to relieve compression on the inferior vena cava (IVC) and the occurrence of vena cava compression syndrome (VCCS). However, the supine position offers advantages for ease of subject positioning and comfort and increased SNR due to optimized coil positioning ⁽¹⁾. The lumbar venous plexus and the bilateral ascending lumbar veins (fig 1) are thought to provide a collateral route for venous return in the supine position to maintain haemodynamic homeostasis ^(2,3).To date, no study has assessed venous return in the IVC and collateral pathways in pregnant women lying supine compared to the left lateral position. MR Phase Contrast imaging was used to assess venous return and its relationship to (a) IVC morphology (b) gestational age (GA), and (c) degree of right-sided positioning of the fundus. We hypothesised 1) that IVC venous return is compromised in the supine position and this is associated with the degree of right-sided positioning of the fundus; and 2) total venous return is generally maintained, providing a protective mechanism against VCCS. Thirteen pregnant participants had imaging of the IVC and collateral pathways in the supine position (GA range 24+0-35+6, median 29+1) with nine participants also imaged in a left lateral tilt (LLT) position (>40⁰), (GA range, 24 +0-35+6, median 28+7). Imaging was performed on a 3T Philips Achieva (Best, Netherlands), using a 32-channel cardiac coil. Written informed consent was obtained prior to imaging. Imaging of the venous system was performed using a peripheral pulse unit (PPU) gated 2D FFE PC-flow (low SAR, low PNS and reduced noise (<110dBA)) with the following parameters: TR=5.7ms, TE=3.5ms, FOV=320x280x2.5mm, slice thickness=8mm, sense factor=2. The venc varied between 50-70cm/s as appropriate to prevent aliasing of the IVC. All imaging was performed perpendicular to the spine at the level of L3/L4 using a breath hold on inspiration for 12 seconds. Blood pressure (BP), heart rate (HR) and oxygen saturation levels were recorded. IVC morphology and the degree of right-sided positioning of the fundus was assessed using ImageJ, v1.49 (National Institute of Health, USA). Morphology of the IVC was estimated by measuring the antero-posterior height of the IVC. PC-flow analysis of the IVC and collateral venous system was carried out using Medviso, Segment v2.0 (http://segment.heiberg.se). Regions of interest (ROI) were drawn around the IVC (fig 2a, green arrow), the bilateral venous plexus and ascending lumbar veins (fig 2b, red circle). Venous return (L/min) was determined by integrating the velocity across space and through time for each ROI in both positions. Statistical analysis was performed using SPSS v21. Paired t-tests were used to assess significant differences in (a) life monitoring, (b) IVC and (c) collateral venous return in the supine versus the LLT position. Linear regression analysis was used to assess the relationship between IVC venous return and (a) GA, (b) degree of right-sided positioning of the fundus and (c) venous return in the collateral pathways. Two participants recorded below normal BP in the supine position. One anxious participant was reassured and successful supine scanning was then carried out with normal BP (100/56). The second, was repositioned by lying left lateral first and then placed supine to move the fundus off the IVC (fig 5), after which normal BP was recorded (100/51) There was no significant difference in life monitoring measures between supine and LLT position (BP (systolic, p=0.63, diastolic, p=0.71), HR (p=0.44), oxygen saturation (p=0.38)), suggesting supine haemodynamic homeostasis was maintained. IVC flow was significantly decreased in the supine position (p=0.018, t=-2.648, dof=8, fig 3a) and was significantly positively correlated to levels of compression in the IVC (r²=0.847, F=38.871, p<0.01, fig 4a) and with the degree of right-sided positioning of the fetus (r²=0.35, F=6.1, p=0.03, fig 4b) but not to GA (r²=0.004, F=0.044, p=0.838). Collateral venous return was significantly higher in the supine position (p=0.01, t=5.6, dof=8, fig3b) and negatively correlated with IVC flow (r²=0.307, F= 4.87, p=0.04, fig 4c). Total venous return (IVC and collateral veins) in both positions showed no significant difference (p=0.99, t=-0.001, dof=8, fig 3c). The LLT did not always show a clear advantage for total venous return compared to the supine position, fig 4d). The spinal venous plexus and the ascending lumbar veins act as a complimentary venous return system to maintain vascular homeostasis in pregnant women lying supine. This supports the proposition that it is feasible to scan pregnant women safely in the supine position with extended life monitoring and particular attention paid to the position of the fundus.