Failing intrauterine support to the fetus can lead to intrauterine growth retardation (IUGR) and hypoxia and it is associated with a high risk of perinatal morbidity and mortality. The main effects of moderate to severe hypoxia on the fetus are different degrees of blood flow redistribution and reduction of oxygen consumption to maintain oxygen delivery to the central organs at the expenses of peripheral organs. The redistribution results in a 'brain sparing' effect. Recently, a Doppler ultrasonic technology (continuous wave, pulsed wave and colour flow imaging) has been developed for the non invasive measurement of flow. Doppler velocimetry detects the flow velocity waveform (FVW) which reflects the cardiac output, the vascular compliance and the resistance to the flow in a defined point of the vessel. Velocity waveform indices or even simpler criteria, such as the presence or absence of diastolic flow or flow reverse during diastole, have been applied to a number of fetal vessels. A significant relationship exists between blood oxygen, systemic lactic acidosis (determined by cordocentesis) and increase PI values in umbilical artery (UA), thoracic aorta (TA) and renal artery (RA). Moreover, in experimental animals during steady state hypoxia, several cardiovascular parameters are affected (heart rate/cardiac output decreases and blood pressure increases) while placental flow don't show a significant variation thus suggesting a raise in placental vascular resistance. Redistribution of the flow may be reliably evaluated by the cerebroplacental ratio (i.e. ratio between PI of MCA and PI of UA, c/p).(ABSTRACT TRUNCATED AT 250 WORDS)