The aim of this study was to evaluate whether pregnancy affects contrast enhancement within the pulmonary arteries during computed tomography pulmonary angiography (CTPA). This was a retrospective analysis of the CTPA examinations of 16 pregnant and 16 non-pregnant female patients, suspected of having an acute pulmonary embolus (PE), during the same time period. Pulmonary vascular enhancement was evaluated by measuring the CT density within the pulmonary arteries. In a blinded evaluation, subjective grading of contrast enhancement within the pulmonary arteries was also performed. There was a significant difference in arterial enhancement between the two groups, with pregnant patients having a mean pulmonary arterial density 112 HU less than patients in the control group [mean attenuation of 259.79 +/- 59.31 HU in pregnant patients versus 371.88 +/- 60.63 HU in non-pregnant patients (p < 0.001)]. The mean subjective pulmonary arterial enhancement score in the pregnant group was 8.19 +/- 2.51 versus 13.69 +/- 3.07 in the control group (p < 0.001). Pregnant women undergoing CTPA have significantly decreased pulmonary arterial enhancement compared to non-pregnant patients, probably due to the increase in cardiac output in pregnancy. We may need to reconsider how we perform CTPA in this group in order to ensure adequate opacification for diagnosis.