It is a common question how a baby receives oxygen while developing inside the womb. Contrary to common assumption, babies do not breathe air through their nose or lungs before birth. Their respiratory system functions differently in the prenatal environment.
Oxygen Delivery in the Womb
A fetus obtains all oxygen and nutrients directly from the birthing parent’s bloodstream. This exchange occurs through the placenta, a specialized organ that forms during pregnancy. The placenta acts as a temporary lung and digestive system for the baby.
Oxygen-rich blood from the birthing parent travels to the placenta, where it transfers to the baby via the umbilical cord. This cord contains one vein that carries oxygenated blood to the fetus and two arteries that return deoxygenated blood and waste back to the placenta. Fetal lungs are filled with fluid and are not involved in gas exchange during this period.
Fetal “Breathing” Movements Explained
Fetuses do not breathe air, but they exhibit “breathing” movements in the womb. These rhythmic movements of the diaphragm and chest muscles are not for gas exchange but serve as practice for breathing after birth. They involve the inhalation and exhalation of amniotic fluid, not air.
Fetal breathing movements begin around 10 to 11 weeks of gestation, becoming more regular as pregnancy progresses. By 24 to 28 weeks, a fetus may spend 10 to 20 percent of its time performing these movements, increasing to about 30 percent by 30 weeks. This practice helps strengthen respiratory muscles and promotes lung development.
Developing Lungs for the Outside World
The development of the respiratory system begins around week 4 of gestation. Lungs undergo several stages of development, from the formation of lung buds to the branching of airways. Milestones include the formation of bronchioles by 18 weeks and the appearance of tiny respiratory sacs, called saccules, around 26 weeks.
These saccules eventually mature into alveoli, the tiny air sacs where gas exchange will occur after birth. Cells within these sacs begin producing surfactant, a substance that reduces surface tension and prevents the alveoli from collapsing. While lungs are considered developed by 37 weeks, alveoli continue to mature and multiply even after birth, into early childhood.
The First Breath of Life
The transition from the fluid-filled womb to the air-filled world is a physiological event for a newborn. At birth, as the umbilical cord is clamped, the baby no longer receives oxygen from the placenta. This triggers changes in the baby’s circulatory system.
The baby’s first breath, often a gasp or cry, inflates the lungs and helps clear the amniotic fluid. This inflation decreases resistance to blood flow in the lungs, allowing blood to flow to them for oxygenation. Simultaneously, the increase in oxygen and changes in pressure cause fetal shunts, such as the foramen ovale and ductus arteriosus, to close, rerouting blood flow to establish independent pulmonary and systemic circulation.