Aortic compression by the uterus: identification with the Finapres digital arterial pressure instrument.

Aortic compression by the uterus at term was diagnosed when reversible reductions in toe pulse pressure, measured using a Finapres digital blood pressure instrument, were found in the absence of systemic hypotension, monitored by a second instrument with the cuff on a finger. Aortic compression was found in the left or right pelvic tilt positions in 14 of 32 women, sometimes with up to 34 degrees of tilt, and was relieved by increasing the amount of tilt, except in a twin pregnancy where compression became more severe as left pelvic tilt was increased from 13 degrees to 28 degrees. Compression was also found in two women in the semi-recumbent position. Factors associated with an increased likelihood of aortic compression were non-engagement of the fetal head, occipito-posterior position of the fetal head, and cervical dilatation less than 5 cm.