Dear Macella,
During late pregnancy, lying on the back (the supine position) can cause the enlarged uterus to press on the inferior vena cava (IVC), the largest vein in the body, which carries deoxygenated blood from the lower half of the body to the heart. This compression reduces blood flow back to the heart and can trigger a condition known as Supine Hypotensive Syndrome.
Supine Hypotensive Syndrome
This syndrome occurs in late pregnancy when a woman lying on her back experiences a sudden drop in blood pressure. Because the IVC is compressed, the amount of blood returning to the heart is reduced, which in turn decreases cardiac output (the volume of blood pumped by the heart each minute).
This can cause symptoms such as dizziness, light-headedness, and fainting.
Treatment
The condition is usually corrected by changing the woman’s position. The recommended position is lying on the left side with knees slightly bent and the body supported by a flat surface — also known as the left lateral recumbent position. This shifts the uterus off the vena cava, restoring normal blood flow.
It is worth noting that this condition does not usually occur during labour. This is because each contraction squeezes blood out of the uterus into the mother’s circulation (a process known as autotransfusion), which increases venous return and helps prevent supine hypotension during that time.
