The question of how a developing human obtains oxygen inside the fluid-filled environment of the womb is often misunderstood. Fetuses do not breathe air or use their lungs for gas exchange while submerged in amniotic fluid. Instead, the entire respiratory function is outsourced to a temporary organ connected to the mother. This highly efficient system, relying on specialized anatomy and a unique circulatory arrangement, ensures continuous support for growth and development until birth.
The Placenta Oxygen Delivery System
The placenta serves as the fetus’s respiratory organ, performing the gas exchange function that the lungs cannot yet execute. This organ grows attached to the wall of the uterus, acting as a biological interface between the mother’s and baby’s circulatory systems. Oxygenated maternal blood flows into a region of the placenta called the intervillous space.
The transfer of oxygen and carbon dioxide across this boundary occurs through simple diffusion. Fetal capillaries, housed within tiny projections called chorionic villi, lie extremely close to the maternal blood, separated only by a thin placental barrier. Oxygen moves from the mother’s higher concentration blood to the fetal blood. Fetal hemoglobin has a higher affinity for oxygen than adult hemoglobin, which enhances the efficiency of this transfer. Carbon dioxide, the fetus’s waste product, diffuses in the opposite direction into the mother’s blood. Once enriched with oxygen, the blood travels from the placenta to the fetus via the umbilical vein.
How Fetal Circulation Bypasses the Lungs
Because the lungs are collapsed and filled with fluid, they present a high resistance to blood flow. The fetal cardiovascular system uses temporary bypasses, known as shunts, that divert blood away from the lungs. This specialized arrangement prioritizes the most oxygen-rich blood for the developing brain and heart.
The foramen ovale is a primary shunt, an opening between the heart’s upper chambers (the right and left atria). Highly oxygenated blood entering the right atrium mostly flows directly through this opening to the left atrium, bypassing the right ventricle and pulmonary circulation. Blood that enters the right ventricle encounters the second shunt, the ductus arteriosus. This vessel connects the pulmonary artery directly to the aorta, shunting most blood away from the lungs and into the systemic circulation. Only a small amount of blood flows through the lungs, sufficient for tissue maintenance and growth but not for respiratory function.
Practicing Breathing Fetal Movements
Fetuses engage in regular, rhythmic contractions of the diaphragm and chest wall, known as fetal breathing movements, despite not using their lungs for gas exchange. These movements, which involve drawing small amounts of amniotic fluid into the lungs and pushing it back out, can be observed on ultrasound as early as 15 weeks of gestation. They become more frequent later in pregnancy.
The purpose of these practice movements is to condition the muscles and structure necessary for breathing after birth, not to obtain oxygen. Regular fluid movement promotes lung growth and the development of chest wall muscles. These movements also help prepare the central nervous system, training the respiratory control centers in the brain. The frequency of these episodic movements, often occurring 30 to 40% of the time late in gestation, is an important indicator of fetal well-being.
The Critical Transition at Birth
The moment a baby is born marks the transition of the circulatory system from a fetal to an adult pattern. This change begins when the umbilical cord is clamped, immediately removing the low-resistance placental circulation and causing a sharp increase in the baby’s systemic blood pressure.
The baby’s first gasp of air fills the lungs with oxygen, causing a dramatic decrease in pulmonary vascular resistance. This sudden drop allows blood to flow freely into the lungs for the first time. The increased blood returning from the functional lungs to the left atrium causes the pressure in the left side of the heart to exceed the pressure in the right side. This pressure shift pushes a flap of tissue across the foramen ovale, causing its functional closure almost immediately. Over time, the foramen ovale seals permanently, becoming the fossa ovalis.
The sudden increase in blood oxygen content and the loss of placental prostaglandins trigger the muscular walls of the ductus arteriosus to constrict. This constriction diverts blood flow fully to the lungs and functionally closes the shunt within the first hours or days of life. The ductus arteriosus eventually closes anatomically and becomes a ligament called the ligamentum arteriosum, completing the shift to a normal adult circulatory pattern.