A healthy heart has four chambers: two atria and two ventricles. However, some babies are born with a misshapen heart. These babies have one normal ventricle and one small or malformed ventricle. Often, they also have a hole between the left and right sides of the heart, allowing deoxygenated blood and oxygenated blood to mix. Fortunately, these kinds of defects — known as single-ventricle defects or SVDs — are rare. Babies born with an SVD can be treated through a sequence of surgeries, as shown in this animation. Surgeons are also developing a pioneering technique to start treatment from within the womb, in the hope of saving even more lives.