Physiological adaptation of the growth-restricted fetus.

The growth-restricted fetus in utero is exposed to a hostile environment and suffers undernutrition and hypoxia. To cope with the stress, the fetus changes its physiological functions. These adaptive changes aid intrauterine survival; however, they can lead to permanent functional and structural changes that can contribute to the development of serious chronic diseases later in life. Epigenetic mechanisms are an important part of the pathophysiological processes behind this "developmental origin of adult diseases." The dominant cardiovascular adaptive change is the redistribution of blood flow in hypoxic fetuses, with preferential supply of blood to the fetal brain, myocardium, and adrenal glands. The proportion of blood from the umbilical vein to the ductus venosus and foramen ovale increases, which increases the cardiac output of the left heart ventricle. The increased perfusion of fetal brain can be followed with Doppler ultrasound as increased diastolic velocities and decreased pulsatility index in the middle cerebral artery.