A pregnant lady complaints of headache, feeling of excessive tiredness and drowsiness whenever she lies down in supine position. Her symptoms are relieved on sitting up and while lying on her side. What is the most likely diagnosis?

Patient is showing features of supine hypotensive syndrome caused by the compression of inferior vena cava by the weight of a pregnant female's uterus, fetus, placenta and amniotic fluids while lying in the supine position. This condition can develop as early as the second trimester but is maximal during the third trimester, 36 to 38 weeks. When the pregnant female in these stages lies in the supine position, the added weight of the enlarged uterus and its contents will press down on the inferior vena cava. When the IVC is compressed it reduces the amount of blood that returns to the hea and reduces cardiac output, this can result in a loss of 30% of the effective circulating blood volume. It can take 3-7 minutes for significant hypotension to manifest.