What is fetal placenta circulation?

Fetal Placental Circulation

The fetal placental circulation is a specialized vascular system that facilitates the exchange of oxygen, nutrients, and waste products between mother and fetus through a complex network of vessels without direct mixing of maternal and fetal blood.

Structure of the Fetal Placental Circulation

Umbilical Cord Vessels

• Two umbilical arteries carry deoxygenated blood from the fetus to the placenta
• One umbilical vein returns oxygenated blood from the placenta to the fetus
• The umbilical vein contains approximately 80-85 mL/kg of fetal blood volume at term, decreasing from about 110-120 mL/kg in the second trimester 1

Placental Vascular Architecture

• The umbilical arteries branch into chorionic arteries on the fetal surface of the placenta
• These vessels further divide into villous arteries that penetrate into the placental tissue
• The villous arteries branch extensively to form arterioles and eventually capillary networks within the terminal villi
• Fetal capillaries within the villi are separated from maternal blood by the placental membrane (syncytiotrophoblast, cytotrophoblast, and basement membrane)
• Capillaries converge to form venules, then villous veins, which eventually form the umbilical vein

Functional Aspects of Fetal Placental Circulation

Blood Flow Characteristics

• Approximately one-third of the fetal cardiac output flows through the placenta at term 1
• Blood flow through the placenta increases progressively throughout pregnancy
• The proportion of fetal blood residing in the placenta decreases from approximately 50% at 30 weeks to about one-third at term 1

Exchange Mechanisms

• Oxygen and nutrients diffuse from maternal blood into fetal capillaries
• Carbon dioxide and waste products diffuse from fetal blood into maternal circulation
• Exchange efficiency depends on:
  • Surface area of villous capillaries
  • Blood flow rates in both maternal and fetal compartments
  • Concentration gradients of substances being exchanged
  • Thickness and permeability of the placental membrane

Vascular Resistance and Regulation

• Umbilical artery resistance normally decreases as pregnancy progresses 2
• Abnormal resistance patterns in the umbilical artery (absent or reversed end-diastolic flow) indicate placental dysfunction and are associated with fetal growth restriction 2
• The fetal circulation demonstrates a "brain-sparing" effect when placental insufficiency occurs, with preferential blood flow to vital organs 2

Pathophysiology in Placental Disorders

Fetal Growth Restriction (FGR)

• Suboptimal perfusion of maternal placental circulation is the most common cause of FGR, accounting for 25-30% of cases 2
• FGR is associated with abnormal umbilical artery Doppler waveforms, reflecting increased placental vascular resistance 2
• Severe FGR shows progression from decreased diastolic flow to absent end-diastolic velocity (AEDV) and eventually reversed end-diastolic velocity (REDV) in the umbilical artery 2

Twin-Twin Transfusion Syndrome (TTTS)

• Occurs in monochorionic diamniotic twin pregnancies due to unbalanced placental vascular anastomoses
• Three main types of anastomoses exist: arteriovenous (AV), arterioarterial (AA), and venovenous (VV) 2
• AV anastomoses (found in 90-95% of monochorionic placentas) can result in unidirectional flow from one twin to the other 2
• AA and VV anastomoses are bidirectional and may compensate for unbalanced flow through AV connections 2

Clinical Assessment of Fetal Placental Circulation

Doppler Ultrasound Evaluation

• Umbilical artery Doppler is the mainstay for assessment of placental circulation 2
• Middle cerebral artery Doppler helps identify brain-sparing effect in compromised fetuses 2
• Cerebroplacental ratio (CPR) = Middle cerebral artery PI / Umbilical artery PI
  • Decreased CPR indicates redistribution of blood flow and is associated with adverse outcomes 2

Management Based on Circulatory Assessment

• Timing of delivery recommendations based on umbilical artery Doppler findings 2:
  • Normal end-diastolic flow with EFW 3-10th percentile: Deliver at 38-39 weeks
  • Decreased end-diastolic flow: Deliver at 37 weeks
  • Absent end-diastolic velocity: Deliver at 33-34 weeks
  • Reversed end-diastolic velocity: Deliver at 30-32 weeks

Clinical Implications and Pitfalls

Important Considerations

• Accurate assessment of gestational age is crucial for proper interpretation of fetal growth and placental function 2
• Placental circulation assessment should be integrated with other fetal surveillance methods (biophysical profile, amniotic fluid assessment)
• Fetal growth assessment should be performed no more frequently than every 2 weeks, with 3-4 weeks being optimal for reliability 2

Common Pitfalls

• Failure to recognize the significance of abnormal umbilical artery Doppler findings
• Over-reliance on a single parameter rather than integrating multiple assessment methods
• Not considering maternal conditions that may affect placental circulation
• Inadequate follow-up frequency in high-risk pregnancies

Understanding fetal placental circulation is essential for identifying abnormal placentation and recognizing the significance of placental pathology in pregnancy complications, which directly impacts fetal and maternal outcomes.