Oxygen Delivery Before Birth
Babies in the womb do not breathe air to obtain oxygen. Instead, their oxygen supply comes entirely from their mother. The placenta, which develops during pregnancy, serves as the interface for this exchange. It attaches to the wall of the uterus and connects to the baby via the umbilical cord.
The placenta contains a vast network of blood vessels from both the mother and the fetus, positioned very closely without directly mixing blood. Oxygen and nutrients from the mother’s bloodstream pass across this thin barrier into the baby’s blood. Simultaneously, waste products like carbon dioxide from the baby’s blood transfer back to the mother’s circulation. The umbilical cord, containing blood vessels, acts as the transport system, carrying oxygenated blood from the placenta to the baby and deoxygenated blood back.
Fetal Practice Breathing
Even though they do not breathe air, babies in the womb exhibit “breathing” movements, starting around 10 to 11 weeks of gestation. These movements are not for oxygen intake but are practice sessions that prepare the respiratory system for life outside the womb. During these episodes, the fetus “inhales” and “exhales” small amounts of amniotic fluid. This fluid moves in and out of the developing lungs, exercising the chest muscles and the diaphragm.
This movement of amniotic fluid through the respiratory system aids lung development by strengthening respiratory muscles and ensuring clear pathways. This developmental exercise is for muscle coordination and lung maturation, not for gas exchange.
Preparing Lungs for Air
The lungs of a developing fetus undergo significant maturation throughout pregnancy in preparation for breathing air. Initially, the lungs are solid structures, but they gradually develop a complex branching system of airways. These airways terminate in tiny air sacs known as alveoli, which are the primary sites of gas exchange after birth. The number and complexity of these alveoli increase as pregnancy progresses.
A key component in this preparation is the production of surfactant, a substance composed of lipids and proteins. Surfactant begins to be produced by specialized cells in the alveoli, called type II pneumocytes, starting around 20 weeks of gestation, with sufficient amounts present by 35 weeks. This substance reduces the surface tension within the alveoli, preventing them from collapsing and sticking together when they inflate with air after birth. Throughout gestation, the fetal lungs remain filled with amniotic fluid, not air.
The First Breath
The moment of birth triggers a series of physiological changes that enable a baby to breathe independently. As the baby passes through the birth canal during a vaginal delivery, chest compression helps squeeze out fluid from the lungs. Any remaining fluid is rapidly absorbed into the baby’s lymphatic system and bloodstream.
Several factors prompt the baby’s first gasp of air. Sudden exposure to a cooler environment outside the womb provides a sensory stimulus. The cessation of the placental oxygen supply, as the umbilical cord is clamped, causes a natural drop in blood oxygen levels and a rise in carbon dioxide. This signals the brain’s respiratory center to initiate breathing. This combination of sensory input and chemical changes in the blood leads to the baby’s first deep inhalation. With this first breath, the lungs inflate, and the tiny alveoli, now coated with surfactant, expand. Pressure changes within the chest also cause a rapid redirection of blood flow, closing certain circulatory shunts that bypassed the lungs during fetal life, allowing blood to flow through the lungs for oxygenation.