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Proper formation of the fetal circulation is critical for normal growth and development. Fetal cardiac output is ejected in parallel from the right and left ventricles of the heart. Due to the presence of the three shunts: the foramen ovale, ductus venosus, and ductus arteriosus, the distribution of blood flow is complex, favoring the direction of more oxygenated blood to the brain and heart and less oxygenated blood to the placenta for reoxygenation. At the time of birth, physiological mechanisms contribute to closure of the vascular shunts, and to the circulatory regulation of blood flow in specific fetal organs such as the liver, lung, and brain. Altered patterns of blood flow in the fetus may indicate hemodynamic problems, cardiovascular dysfunction, or placental insufficiencies. Doppler ultrasound provides a useful clinical tool for evaluation of fetal complications in the circulation. [ABSTRACT FROM AUTHOR]