Positioning in the pregnant patient is very important. About 10% of women at term experience the supine hypotensive syndrome caused by complete or near-complete occlusion of the inferior vena cava (IVC) by the gravid uterus. A resultant drop in venous return decreases cardiac output (CO), leading to systemic hypotension for which the cardiovascular system cannot compensate. The nature and severity of symptoms range from unspecific symptoms such as pallor, sweating, nausea, and vomiting, to severe maternal hypotension, loss of consciousness, cardiovascular collapse, and consecutive fetal depression. Inferior vena caval compression can begin to occur in pregnant women as early as 13 weeks’ gestation and may be exaggerated by multiple gestations (twins) and obesity.

Complete or near-complete obstruction of the IVC can occur when the parturient is supine. Collateral circulation can allow for some venous return, but the result is a decrease in right atrial filling pressure. This drop in preload leads to a decrease in CO. The lateral decubitus position causes partial caval obstruction, but the right ventricular filling pressure is unaltered due to collateral circulation, and there is little effect on the CO.

Aortic compression also occurs. There is little aortic compression at term in the lateral position. In the lateral tilt position, 40% of patients will have a fall in femoral artery pressure, indicating a compression of the aorta. In the supine position, the fall is even greater and inversely proportional to arterial pressure. Thus, there is enhanced compression of the aorta when the patient is hypotensive.

Objective parameters of fetal well-being deteriorate in the supine position compared to the lateral or tilted position. Neonatal outcome, as accessed by umbilical cord pH and Apgar scores, is also compromised in the supine position. These effects are explained by aortocaval compression and the resulting impairment in uteroplacental blood flow.