Primigravida with full term, complains of faintness on lying down and se feels well when turns to side or sitting position. This is due to:

SUPINE HYPOTENSION SYNDROME (POSTURAL HYPOTENSION): During late pregnancy, the gravid uterus produces a compression effect on the inferior vena cava when the patient is in supine position. In 90% cases this, however, results in opening up of the collateral circulation by means of paravertebral and azygos veins. In some cases (10%), when the collateral circulation fails to open up, the venous return of the heart may be seriously curtailed. This results in production of hypotension, tachycardia and syncope called as supine hypotensive syndrome. The normal blood pressure is quickly restored by turning the patient to lateral position. That is why pregnant females are advised to lie in lateral positions best being left lateral.