The maternal adaptation to pregnancy includes hyperlipidaemia, oxidative stress and chronic inflammation. In non-pregnant individuals, these processes are usually associated with poor vascular function. However, maternal vascular function is enhanced in pregnancy. It is not understood how this is achieved in the face of the adverse metabolic and inflammatory environment. Research into cardiovascular disease demonstrates that plasma HDL (high-density lipoprotein), by merit of its functionality rather than its plasma concentration, exerts protective effects on the vascular endothelium. HDL has vasodilatory, antioxidant, anti-thrombotic and anti-inflammatory effects, and can protect against endothelial cell damage. In pregnancy, the plasma HDL concentration starts to rise at 10 weeks of gestation, peaking at 20 weeks. The initial rise in plasma HDL occurs around the time of the establishment of the feto-placental circulation, a time when the trophoblast plugs in the maternal spiral arteries are released, generating oxidative stress. Thus there is the intriguing possibility that new HDL of improved function is synthesized around the time of the establishment of the feto-placental circulation. In obese pregnancy and, to a greater extent, in pre-eclampsia, plasma HDL levels are significantly decreased and maternal vascular function is reduced. Wire myography studies have shown an association between the plasma content of apolipoprotein AI, the major protein constituent of HDL, and blood vessel relaxation. These observations lead us to hypothesize that HDL concentration, and function, increases in pregnancy in order to protect the maternal vascular endothelium and that in pre-eclampsia this fails to occur.