Supine hypotension is characteristic of the following trimester of pregnancy:

During late pregnancy with the woman in the supine position, the large pregnant uterus rather consistently compresses venous return from the lower body. In about 10 percent of women, this supine compression of the great vessels by the uterus causes significant aerial hypotension, sometimes referred to as the supine hypotensive syndrome. Also when supine, uterine aerial pressure--and thus blood flow--is significantly lower than that in the brachial aery. This may directly affect fetal hea rate patterns. This also occurs with hemorrhage or with spinal analgesia. Ref: Cunningham F.G., Leveno K.J., Bloom S.L., Hauth J.C., Rouse D.J., Spong C.Y. (2010). Chapter 5. Maternal Physiology. In F.G. Cunningham, K.J. Leveno, S.L. Bloom, J.C. Hauth, D.J. Rouse, C.Y. Spong (Eds), Williams Obstetrics, 23e.