Fetal Circulation and Structures

Fetal Circulation

• Fetal oxygen comes from the placenta, not the lungs
• Maternal and fetal circulatory systems connect via the placenta
• The placenta takes metabolic wastes and carbon dioxide from the fetus
• Temporary structures in the fetal circulatory system direct blood away from the lungs and to the placenta via the umbilical cord
• By 3 weeks gestation, the fetal cardiovascular system develops, ensuring intrauterine survival

Temporary Fetal Circulation Structures

• Ductus venosus: Oxygenated blood from the umbilical vein is directed to the inferior vena cava
• Two hypogastric arteries: Deoxygenated blood from the lower extremities is carried back to the placenta via umbilical arteries.
• Foramen ovale: Blood bypasses the pulmonary circulation through the septum between the atria.
• Ductus arteriosus: Blood flow between the pulmonary arteries and the aorta prevents blood from entering the pulmonary circulation.

Fetal Blood

• The placenta is less efficient at respiration than mature lungs.
• Fetal blood cells have unique adaptations for oxygen:
  • Larger red blood cells
  • More red blood cells with higher hemoglobin content
  • Shorter lifespan of fetal hemoglobin (HbF) – 90 days
  • HbF carries oxygen at lower pressures
• Towards the end of pregnancy, HbF is replaced by adult hemoglobin (HbA)

Respiratory System in Utero

• At term, the fetal lungs are well-developed, with over 25% of alveoli present.
• Fetal lungs contain approximately 25 ml/kg of pulmonary fluid.
• Breathing movements begin around 10 weeks gestation and encourage diaphragm and muscle development
• Surfactant, a surface-active lipoprotein secreted by alveolar type II pneumocytes (increasing in amount from 32 weeks), reduces surface tension, facilitating lung expansion after birth.

Neonatal Adaptation at Birth

• Pulmonary Lung Fluid: Absorption of fluid occurs during labor and birth, primarily through lymphatic and pulmonary vessels, returning it into the bloodstream.
• Most newborns initiate breathing within a few seconds of birth and establish regular respiration within minutes.
• Factors that initiate first breath and lung expansion include:
  • Compression of the chest wall
  • Stimulation by lowered oxygen and increased carbon dioxide levels in the blood
  • Sensory stimulation (touch, pressure, decreased temperature)
  • Lung inflation in a newborn is complete within the first few breaths, with most alveoli fully expanded within hours.
  • Lung volume reaches ~25 ml/kg body weight.
• The respiratory center in the medulla oblongata regulates respiratory effort based on metabolic needs.

Neonatal Hematological Changes

• Fetal-placental blood volume varies from 110-130 mL/kg.
• Circulating blood volume averages 85-90 mL/kg of body weight at term.
• Hematopoiesis (blood cell production) is fast and responsive.
• Fetal hemoglobin (HbF) is gradually replaced by adult hemoglobin (HbA) within the first week after birth.
• Neonatal hemoglobin ranges from 150-235 g/L and decreases to roughly 120 g/L by three months.

Delayed Cord Clamping

• Delaying cord clamping allows placental blood to support the volume of blood required during alveolar capillary bed expansion.
• Delaying cord clamping leads to a gentler increase in systemic resistance
• Facilitates smoother transition to extrauterine life, enabling better physiological transfer of placental blood volume (~80-100 mL, including iron).

Assessment of the Baby at Birth (APGAR)

• The APGAR score is useful for structured assessment of extrauterine adjustment.
• The score looks at the baby's condition and level of alertness.
• Scoring is done at 1 minute, 5 minutes and 10 minutes.
• Scores above 7 suggest little intervention is required, while those below 7 may need resuscitation or further support

Ongoing Cardio-Respiratory Assessments

• Evaluation of a newborn involves several factors including history (gestational age, birth mode, feeding history), and risk factors (cardiac, genetic, respiratory).
• Vital signs include color and perfusion, tone, respiratory rate (40-60 bpm), and heart rate (110-150 bpm).
• Observing for signs of respiratory distress (grunting, nasal flaring, sternal recession, ↑RR, ↑HR) is crucial.
• Ongoing assessment requires discussion with the supervisor, consideration of potential referrals, a plan of care, and communication with parents.