Antenatal surveillance

The placenta is a two compartment nutrient, fluid and gas exchange organ:

• The maternal compartment is assessed by uterine artery Doppler

• The fetal compartment is assessed by umbilical artery Doppler

Physiological maturation of the vasculature with advancing gestation is reflected in:

• Uterine arteries: Increase in diastolic flow with loss of the early-diastolic notch
• Umbilical arteries: Appearance of end-diastolic velocity
• Decline in pulsatility index in both vessels

In abnormal placentation there is:

• Impaired trophoblastic invasion of the maternal spiral arteries resulting in high uterine artery pulsatility index and persistence of the uterine artery notch
• Abnormal villous vascular tree resulting in abnormal waveform in the umbilical arteries which depends on the degree of impaired vasculature:
  
  • 30% impairement results in high pulsatility index
  • 50% impairement results in absent end-diastolic velocity
  • 70% impairement results in reversed end-diastolic velocity

  The risk for hypoxemia / acidemia is proportional to the decrease in umbilical end-diastolic flow