Babies do not “breathe” air in the womb as adults do. Their oxygen source is different, as their lungs are not yet functioning for gas exchange. Instead, a specialized system provides them with oxygen and nutrients for growth and development.
Oxygen Delivery in the Womb
A fetus receives oxygen through the placenta, which acts as its respiratory system before birth. The placenta, which develops in the mother’s uterus, transfers oxygen and nutrients from the mother’s bloodstream to the fetus. Oxygenated blood from the mother’s circulation enters the placenta, where gas exchange occurs across its membrane.
From the placenta, oxygen-rich blood travels to the fetus via the umbilical vein. This blood circulates through the fetal cardiovascular system, which has adaptations to bypass the non-functional lungs. Waste products, including carbon dioxide, are transported from the fetus to the placenta through the umbilical arteries, then transferred to the mother’s circulation for elimination.
Fetal Breathing-Like Movements
Babies exhibit “breathing-like movements,” also known as fetal breathing movements, even though they do not breathe air in the womb. These are not true respiratory efforts involving air, but practice movements where the fetus inhales and exhales amniotic fluid. These rhythmic movements, which can begin as early as 10 weeks and become more consistent around week 20, serve a role in development.
These movements strengthen respiratory muscles like the diaphragm and intercostal muscles. They also promote lung development by moving fluid in and out of the developing airways. Their presence indicates normal fetal development and preparation for breathing after birth.
The Process of Lung Maturation
Lung development begins early in pregnancy and continues after birth. The respiratory system starts forming around week 4 with a lung bud. This bud branches repeatedly, forming the network of airways, including the trachea, bronchi, and smaller bronchioles.
Later stages involve the formation of alveoli, tiny air sacs where gas exchange will occur. Type I and Type II alveolar cells differentiate, with Type II cells producing surfactant around 24 to 28 weeks. Surfactant reduces surface tension within the alveoli, preventing collapse when the baby takes its first breath.
The First Breath: Transition at Birth
Birth triggers a physiological transition, as the baby shifts from relying on the placenta for oxygen to breathing independently. Several stimuli contribute to this initial breath, including cooler air, pressure changes, and umbilical cord clamping. The baby’s central nervous system reacts to these environmental changes, initiating the first gasp of air, often within 10 seconds of delivery.
As the baby takes its first breath, the lungs expand, clearing amniotic fluid and filling with air. This expansion causes a rapid decrease in pulmonary vascular resistance, increasing blood flow to the lungs. Simultaneously, fetal circulatory shunts, such as the foramen ovale and ductus arteriosus, begin to close, reconfiguring the circulatory system to direct blood through the now-functioning lungs. This adaptation allows the lungs to take over oxygen exchange outside the womb.