Nursing care
Why supine hypotension occurs in pregnancy and why lateral tilt helps
Written and reviewed by Dana Whitfield, RN, MSN · 4 min read · Updated October 2026
Short answer
From around mid-pregnancy, the heavy uterus can press on the inferior vena cava and the aorta when the patient lies flat on her back. Less blood returns to the heart, so cardiac output and blood pressure fall, and blood flow to the uterus and placenta can drop. Turning to the side, usually the left, moves the uterus off these vessels.
Trace the pressure from the uterus to the heart
The inferior vena cava runs along the right side of the spine and carries blood from the legs and lower body back to the heart. By around 20 weeks of gestation, the enlarging uterus is heavy enough to compress it, along with the descending aorta, when the patient lies supine. Cardiac output becomes increasingly sensitive to position as pregnancy advances.
Compression of the vena cava reduces venous return, so the heart has less blood to pump with each beat. Stroke volume and cardiac output fall. Compression of the aorta can further reduce blood flow to the lower body and the uterus. Because the placenta has no way to regulate its own blood supply, a fall in maternal blood pressure can quickly reduce fetal oxygen delivery.
Why some patients compensate and others do not
Many pregnant patients compensate. Sympathetic tone rises, the heart rate increases, and blood from the lower body is diverted back to the heart through alternative veins around the spine. In these patients compression may be present without obvious symptoms. The compensation can hide reduced uterine blood flow, which is one reason supine positioning is avoided even when the patient feels well.
When compensation fails, supine hypotensive syndrome develops: pallor, sweating, nausea, dizziness, an initial fast heart rate that may become slow, and low blood pressure, all relieved by turning to the side. Anything that blunts sympathetic compensation makes this more likely. Neuraxial anaesthesia, such as an epidural or spinal, and general anaesthesia both reduce the body's ability to defend blood pressure.
Read the maternal and fetal signs together
On the maternal side, look for a patient who becomes faint, pale, clammy or nauseated shortly after lying flat, with a drop in blood pressure. These signs typically improve within minutes of repositioning. On the fetal side, reduced uterine blood flow may show on the monitor as a change in fetal heart rate pattern, such as decelerations or reduced variability.
The expected response to repositioning is a recovering blood pressure, resolving symptoms and an improving fetal heart rate pattern. The concerning picture is hypotension or fetal heart rate changes that persist despite lateral positioning, which suggests another cause such as bleeding, anaesthetic effects or other complications. That situation requires prompt escalation, not just further position changes.
Use positioning as the first nursing action
Avoid the flat supine position in the second half of pregnancy. For examinations, procedures and rest, position the patient on her side or place a wedge under the right hip so the uterus tilts to the left. Left lateral positioning has long been preferred because the vena cava lies to the right, though either side relieves compression better than lying flat.
If hypotension occurs, turn the patient to her side straight away, then reassess blood pressure, pulse, symptoms and the fetal heart rate. Notify the provider or anaesthetist, particularly after neuraxial anaesthesia, where additional fluids or medicines may be prescribed. During resuscitation of a pregnant patient, manual displacement of the uterus to the left is used under resuscitation protocols to relieve compression.
Work through a hypothetical exam-style scenario
Imagine a hypothetical patient at 34 weeks lying flat for an ultrasound who suddenly feels dizzy and nauseated, looks pale and has a lower blood pressure than on arrival. The options are to raise her legs while she stays supine, to start oxygen and call for help while leaving her position unchanged, or to turn her onto her left side. Turning her is the strongest first action.
Raising the legs leaves the uterus on the vena cava, and oxygen without repositioning does not remove the cause. The question tests whether the candidate recognises a mechanical obstruction that a simple position change can correct. After turning her, the nurse reassesses maternal vital signs and fetal status and escalates if they do not improve.
Sources and further reading
PMC: Physiological and anatomical changes of pregnancy: implications for anaesthesia. Compression of the IVC and aorta from about 20 weeks, sympathetic and venous compensation, signs of supine hypotensive syndrome, anaesthesia effects and lateral uterine displacement.
MSD Manual Professional: Physiology of pregnancy. Positional fall in cardiac output when the uterus obstructs the vena cava and relief of great vessel pressure in the left lateral position.
The next step on this is the same as on everything else here: answer questions and read the rationales. Our maternity and newborn practice questions are the closest set to what this page covers.
Common questions
When does aortocaval compression usually begin?
The uterus is typically large enough to compress the vena cava and aorta in the supine position from about 20 weeks of gestation, and the effect becomes more marked later in pregnancy.
Why is the left side preferred?
The vena cava lies to the right of the spine, so tilting the uterus to the left moves it off the vessel. A wedge under the right hip achieves the same goal during procedures.
Why does an epidural make supine hypotension worse?
Neuraxial anaesthesia reduces sympathetic tone, which the body relies on to compensate for reduced venous return, so blood pressure falls more readily when the patient lies flat.