Introduction During healthy pregnancy there is marked and progressive vasodilatation leading to increased blood flow to maternal organs, in particular the utero-placental circulation. While using a measure of flow-mediated dilatation (FMD) to assess endothelial function in pregnancy, we noticed vasoconstriction of the brachial artery during low-flow, induced by distal cuff occlusion. Technological improvements in measuring and analysing vascular responses to shear stress allowed us to quantify these novel changes that are so far, unique to pregnancy.

Methods Endothelial function was prospectively assessed using FMD in 40 healthy, non-smoking pregnant women from 10 weeks gestation, at 4–6 weekly intervals throughout pregnancy and at 15 weeks postpartum. The distal brachial artery was occluded for 5 min and brachial artery diameter was measured continuously for 1 min prior to occlusion, during vessel occlusion and for 5 min after cuff release. Maternal heart rate was measured during this time, and blood flow and reactive hyperaemia responses were calculated.

Results A novel and unexpected observation was vasoconstriction of the brachial artery at low-flow, proximal to cuff occlusion. As peripheral blood flow increased, this vasoconstriction became more marked with advancing gestation (ANOVA p<0.001) and fell postpartum. As previously observed, maternal heart rate increased and reactive hyperaemia decreased as pregnancy advanced (ANOVA p<0.001 in both cases). FMD was unchanged during pregnancy but fell postpartum.

Conclusions Reducing peripheral blood flow in pregnancy leads to proximal artery vasoconstriction. This observation supports the view that high flow shear stress mediates at least part of the progressive vasodilatation of healthy pregnancy.