Brain sparing is a physiological mechanism used by the fetus to increase delivery of oxygenated blood to the brain at the expense of other organs. Unlike fully formed adults, only three organs are of importance to the fetus: the brain, the heart, and the adrenals (glands which produce adrenaline and natural steroids). When a fetus senses that the amount of oxygen in its blood is decreasing it makes an effort to protect its three most vital organs at the expense of all others, such as the liver, kidneys, muscles etc. This is achieved by a sophisticated cardiovascular response, which diverts more well-oxygenated blood to the important organs. This mechanism can protect the baby for up to 3 weeks.