GYNECOLOGY &OBSTETRICS International Journal of Gynecology & Obstetrics 55 (1996) 213-218 Article Influence of compression of the inferior vena cava in the late second trimester on uterine and umbilical artery blood flow E. Rye*, T. Okai, S. Kozuma, K. Kobayashi, A. Kikuchi, Y. Taketani Depatiment of Obstettics and Gynecology, Faculty of Medicine, University of Tokyo, Tokyo, Japan Received 25 April 1996; revised 15 July 1996; accepted 17 July 1996 Abstract Object&e: To evaluate the inferior vena cava compression and its influence on the uterine and umbilical artery blood flow in the late second trimester when the mother is supine. Methods: The inferior vena cava diameter was measured by ultrasound B mode scan, and Doppler flow velocimetry of the uterine and umbilical artery was performed in 90 women at 24-27 weeks in the supine and complete left lateral position. Results: The inferior vena cava diameter in the supine position was significantly smaller than that in the lateral position. The degree of the vena cava compression was associated with an elevation in the uterine artery RI in the supine position. The umbilical artery RI did not couple with either the degree of the compression or the changes in the uterine artery RI. Conclusion: The inferior vena cava is compressed in the majority of pregnant women in the second trimester, and the compression may affect the uterine artery blood flow but not the fetal circulation. Copyright 0 1996 International Federation of Gynecology and Obstetrics. Keywords: Inferior vena cava compression; Supine position; Uterine artery; Umbilical artery; Doppler velocimetry; Second trimester 1. Introduction The supine hypotensive syndrome, which often occurs in the third trimester of pregnancy, is *Corresponding author. Tel.: + 81 3 38155411 (ext. 3407); fax: +81 3 38162017. thought to be a consequence of the compression of the inferior vena cava by the large pregnant uterus when supine. The syndrome is associated with significant arterial hypotension with the inci- dence in the range between 2.5 and 20.6% [l]. Considering that the syndrome is caused by the reduced cardiac filling due to the compression of the venous system, it follows that the compression 0020-7292/96/$15.00 Copyright 0 1996 International Federation of Gynecology and Obstetrics PII SOO20-7292(96)02760-9