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

Supine hypotension syndrome : During late pregnancy,the gravid uterus produces a compression effect on the IVC when patient is in supine position . In 90% cases, this results in opening up of the collateral circulation by means of paraveebral and azygos veins. In 10 % cases, when collaterals fails to open up, the venous rReturn Ofthe hea may be seriously cuailed. This results in production of hypotension, tachycardia and syncope -Supine Hypotension Syndrome. The normal blood pressure is quickly restored by turning the patient to lateral position. Ref:Datta Obs 9e pg 49.