Fetuses do not breathe air in the womb in the same way that infants and adults do. The environment within the uterus, filled with amniotic fluid, necessitates a different mechanism for obtaining oxygen and expelling carbon dioxide. This unique fetal physiology supports growth and development until birth, when a dramatic transition to air breathing occurs.
Oxygen Delivery in the Womb
The fetus receives all necessary oxygen through the placenta, an organ that develops during pregnancy. The placenta acts as the primary site for gas exchange between the mother and the fetus. Oxygen from the mother’s oxygen-rich blood diffuses across the placental membrane into the fetal capillaries. Simultaneously, carbon dioxide, a waste product, diffuses from the fetal blood back into the mother’s circulation.
The umbilical cord serves as the vital connection, transporting blood between the fetus and the placenta. It contains one umbilical vein and two umbilical arteries. The umbilical vein carries oxygenated, nutrient-rich blood from the placenta to the fetus. Conversely, the umbilical arteries carry deoxygenated blood and waste products from the fetus back to the placenta. This system ensures a continuous supply of oxygen to developing fetal organs, bypassing the lungs, which are not yet functional for gas exchange.
Fetal Breathing Movements
While fetuses do not breathe air, they engage in rhythmic “breathing movements.” These movements involve contractions of the diaphragm and chest muscles, leading to the inhalation and exhalation of amniotic fluid. These movements are distinct from actual respiration, as they do not facilitate oxygen uptake. Their primary purpose is developmental, helping to strengthen respiratory muscles and promote the growth and maturation of the fetal lungs.
Fetal breathing movements begin around 10 to 11 weeks of gestation, becoming more regular and pronounced after about 28 weeks. Moving amniotic fluid in and out of the developing lungs is crucial for the expansion of lung tissue and the production of surfactant. Surfactant is a substance that reduces surface tension in the lungs, preparing the alveoli to remain open after birth. These practice breaths are an important marker of fetal well-being and prepare the lungs for their eventual function outside the womb.
The Transition to Air
At birth, a series of rapid physiological changes occur, enabling the newborn to breathe air independently. As the umbilical cord is clamped, the placenta’s role in oxygen supply ceases, and systemic vascular resistance increases. The first breaths the baby takes are forceful, helping to clear the amniotic fluid from the lungs. This fluid clearance is largely mediated by the lung’s epithelial cells switching from fluid secretion to active fluid absorption.
The expansion of the lungs with air causes a decrease in pulmonary arterial pressure and resistance. This allows a larger volume of blood to flow through the lungs, facilitating gas exchange. Simultaneously, changes in pressure within the heart chambers, along with increased oxygen tension, trigger the closure of fetal circulatory shunts. The baby’s first cry accompanies these initial breaths, marking the transition to independent respiratory function.