Before birth, a baby’s lungs are not used for breathing air. Instead, life is sustained through the placenta, which acts as a natural life support system.

Inside the placenta, oxygen from the mother’s blood crosses a thin barrier and enters the baby’s blood. Fetal hemoglobin, a special protein in red blood cells, captures oxygen more efficiently than adult hemoglobin, ensuring the baby always receives what it needs.

Once oxygen is captured, it travels through the umbilical cord, carrying oxygen-rich blood from the placenta into the baby’s body. From there, the baby’s circulation distributes oxygen to every developing cell.

At the same time, carbon dioxide and waste products flow the opposite way. They leave the baby’s blood, cross back into the placenta, and enter the mother’s circulation. Her lungs exhale the carbon dioxide, and her kidneys and liver process the other wastes.

Although the lungs are not yet breathing air, they are not idle. Filled with fluid, they make tiny “practice” breathing movements that prepare the muscles and structures for the outside world.

When birth finally comes, the first breath fills the lungs with air, the placenta’s work is finished, and the umbilical cord can be cut. Until that moment, life depends on the placenta and umbilical cord together, a shared circulation that sustains growth until independence begins with a cry.

How does the baby know to breathe?
The trigger comes from a combination of changes. As the placenta stops delivering oxygen, carbon dioxide builds up in the baby’s blood. This rising carbon dioxide stimulates the respiratory center in the brainstem, which automatically sends signals to the diaphragm and chest muscles to contract. At the same time, the sudden exposure to cooler air and gentle physical stimulation from touch strengthen the response. 

The result is a powerful first inhale, often followed by a cry, which expands the lungs and marks the start of independent life.