A fetus does not breathe air in the womb. It lives in a unique, aquatic environment inside the uterus where oxygen exchange occurs through a completely different physiological mechanism. This intrauterine period is characterized by adaptations that allow for growth and development without the need for functional lungs, preparing the fetus for the dramatic shift that occurs at birth.
Oxygen Delivery Through the Placenta
The fetus receives all necessary oxygen and nutrients from its mother via the placenta, which acts as the body’s temporary respiratory and digestive organ. This specialized structure connects to the fetus through the umbilical cord, a lifeline containing blood vessels that manage the exchange process. Oxygenated blood from the maternal circulation is delivered to the placenta’s exchange surface, bypassing the fetus’s own lungs.
The transfer of oxygen and carbon dioxide across the placental barrier occurs through simple diffusion. Maternal blood and fetal blood flow in close proximity, but they do not mix, allowing oxygen molecules to move down their concentration gradient into the fetal circulation. Deoxygenated blood and metabolic waste products, such as carbon dioxide, move in the opposite direction, passing from the fetal blood back into the mother’s bloodstream for her body to eliminate. This continuous, passive exchange is how the fetus sustains life while suspended in amniotic fluid.
Fetal Breathing Movements
Although the fetus does not inhale air, it regularly performs rhythmic muscle contractions known as Fetal Breathing Movements (FBMs). These movements involve the diaphragm and chest wall, mimicking the action of breathing but resulting only in the movement of amniotic fluid into and out of the lungs. FBMs begin early in gestation and become more prominent in the later stages of pregnancy.
The purpose of these movements is not gas exchange, but rather to strengthen the muscles that will be used for actual respiration after birth. FBMs also play a significant role in promoting the growth and development of the lungs themselves. By moving fluid, these practice sessions help to regulate the volume of lung fluid and aid in the development of the respiratory center in the brain. They are intermittent, with the fetus spending between 30 to 40 percent of the time performing these movements in the third trimester.
Preparing the Lungs and Fetal Circulation
The lungs must undergo extensive structural preparation to function in the air, even while they are filled with fluid in utero. A substance called surfactant is produced by specialized cells within the lungs, beginning around the sixth month of pregnancy. This fatty substance coats the inner surfaces of the air sacs, or alveoli, and its function is to lower surface tension, preventing the delicate air sacs from collapsing once they inflate with air after birth.
To keep blood flow away from the non-functional lungs, the fetal circulatory system employs unique bypass structures called shunts. The foramen ovale allows blood to flow directly from the right atrium to the left atrium, bypassing the right ventricle and the lungs. The ductus arteriosus connects the pulmonary artery directly to the aorta, diverting blood away from the lungs and into the systemic circulation. These diversions ensure that the most oxygenated blood is preferentially delivered to the developing brain and heart, which are the most oxygen-sensitive organs.
The Physiological Transition at Birth
The moment of birth triggers a rapid and profound physiological shift, converting the fetal circulatory and respiratory systems into neonatal systems. The first sequence of events often begins with the compression of the chest during passage through the birth canal, which helps to squeeze a significant portion of the fluid from the lungs. The subsequent expansion of the chest, coupled with the baby’s first cry, creates a massive negative pressure that draws air into the lungs, replacing the remaining fluid.
This influx of air causes the blood vessels in the lungs to open, resulting in a drop in pulmonary vascular resistance. Simultaneously, clamping the umbilical cord removes the low-resistance placental circulation, leading to an immediate increase in the baby’s systemic vascular pressure. The combination of increased oxygen levels and the loss of placental prostaglandins causes the muscular wall of the ductus arteriosus to constrict and close. This rapid redirection of blood flow increases the pressure in the left side of the heart, causing the flap-like foramen ovale to shut, completing the separation of the heart’s right and left sides.