What is the last fetal system to mature functionally?

The respiratory system is the last fetal system to mature functionally. During gestation, while other systems such as the cardiovascular and nervous systems develop relatively early, the respiratory system requires significant development primarily in the later stages of pregnancy. This is because the fetus relies on the placenta for gas exchange until birth.

Around 24 to 28 weeks of gestation, the lungs begin to produce surfactant, a substance that reduces surface tension within the alveoli, allowing them to expand properly after birth. Surfactant production increases significantly as the pregnancy progresses, peaking in the final weeks before delivery, which is crucial for the newborn’s ability to breathe air and maintain adequate oxygenation. Therefore, the functional maturation of the respiratory system is vital for transitioning from fetal to neonatal life.

Understanding this timeline highlights why other systems, like the cardiovascular and renal, develop earlier and are ready to carry out their functions before birth, while the respiratory system's maturation directly correlates with the demands of transitioning to air breathing at the time of delivery.