Supine hypotension is a characteristic feature of:

Supine Hypotension syndrome: During late pregnancy, the gravid uterus produces a compression effect on the inferior vena cava when the patient is in supine position. This however results in the opening up of the collateral circulation by means of paraveebral and azygos veins. In some cases when collateral circulation fails to open up, the venous return to the hea may be seriously cuailed. This results in production of hypotension, tachycardia and syncope. Note: The normal blood pressure is quickly restored by turning the patient to lateral position. The augmentation of the venous return during uterine contraction prevents the manifestation from developing during labour. Ref: Textbook of Obstetrics D.C.Dutta 6th Ed Page 53.