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Labor & delivery

Physiologic changes in the newborn

Going from the womb to the outside world is a challenging journey for both mother and baby. Both undergo a variety of physical changes that make this transition possible. The most important adaptation, as well as the most sophisticated, is the baby’s transition to breathing air. In order to accomplish this, the baby must undergo changes in circulation, heart anatomy, lung function, and hemoglobin structure.

In order to breathe air, the circulatory system must start delivering a great deal of blood to the lungs. The increased blood to the lungs means that less blood is sent to the rest of the body. This disproportionate need for blood in the lungs stresses the heart to pump harder. As this happens, other changes are taking place in the heart. Two small openings in the newborn heart, the foramen ovale and the ductus arteriosus, begin to close. This closing completes during the first few days of life.

Fetal hemoglobin is the part of the fetal red blood cell that is responsible for delivering oxygen to all areas of the body. Fortunately, the fetal hemoglobin does a very efficient job of attracting circulating oxygen in the blood – because the fetus is not an air breather. After the baby is born, they begin to breathe air, and this signals development of a new type of hemoglobin. This is one of many physical changes that a newborn undergoes as they adjust to their new environment.