Doppler principle states that when energy is reflected from a moving boundary, the frequency of the reflected energy varies in relation to the velocity of the moving boundary.
Clinically, this principle is used to determine the velocity of blood flow in vessels. The frequency of sound reflected off moving blood cells is slightly altered from the sound emitted from the source (the trasducer) in proprortion to the velocity of the blood. This frequency shift can be determined by currently available technology and converted to velocity of blood flow that appear on the screen as a wave with a peak, the Systole, and a slope, the Diastole, before a new peak.
Doppler sonography in obstetrics and gynecology can be performed using either the transabdominal or endovaginal route.

One of the main goals of prenatal testing is to identify fetuses at increased risk for perinatal morbidity and mortality. Fetal hypoxia and asphyxia, often combined with intrauterine growth retardation (IUGR), is associated with significantly incrased risk. Therefore, much of the interest in Doppler sonography has focused to identify the IUGR, hypoxic, and/or distressed fetus.
Because prenatal Doppler studies have not been proved to benefit pregnancy outcome, many investigators still consider obstetric Doppler velocimetry to be an investigational tool. If Doppler imaging has a role in obstetrics, the majority of data suggest that it may be more useful in the higher-risk population.

Maternal side of the placental circulation
Both the uterine and fetal circulation have been studied with Doppler sonography. The uterine arteries are two symmetrical maternal vessels that bring the well oxygenated blood of the mother to the placenta. There is little diastolic flow in the uterine artery in the nongravid uterus. As normal gestation progresses, there is a progressive drop in impedance in the uterine artery through the second trimester, as evidenced by increased diastolic flow velocity with Doppler scanning. Thereafter, resistance remains stable.
Abnormal waveform patterns may be associated with

  1. maternal conditions that impair blood supply to the placenta and are associated with impaired fetal growth
  2. abnormal uterine artery waveforms have been correlated with current or future development of maternal hypertension.

Independent of pregnancy-induced hypertension, abnormalities or uteroplacental waveforms have been observed in association with impaired fetal growth, although the results have been inconsistent.

Fetal side of the placental circulation
Alteration in the fetoplacental circulation and specific fetal vessels reflect important hemodynamic modification that occur in association with IUGR and fetal hypoxemia. The Doppler-detectable modifications in the fetal circulation associated with IUGR and fetal hypoxemia include increased resistance in the umbilical artery and fetal peripheral vessels , in association with decreased resistence in the fetal cerebral vessels. This is the "brain-sparring" phenomenon, in which fetuses that are rendered hypoxic preferentially perfuse the brain, heart, and adrenals at the expense of the integument and viscera, gut, and kidneys.