While in the womb, a fetus’s oxygen needs are met by the mother. Blood is pumped by the baby’s heart through the umbilical cord arteries to the placenta where oxygen is extracted from the mother’s blood as it passes by the placenta. The maternal and fetal blood systems are separate. It is only oxygen that passes from maternal blood vessels to placental blood vessels by a process called diffusion. The umbilical cord vein delivers oxygenated blood to the fetus’s heart, where it is then pumped to the rest of the body.
In the womb, the fetus’s lungs are filled with fluid. They are not used for breathing, but are developing and getting ready for baby’s first breath after birth. Compared to other organs such as the heart and kidneys, which are already functional in the womb, the lungs will not start working until after birth and are slower to develop to maturity. By the 24th week of pregnancy or so, most of the basic parts of the lung have developed, but it is not until the 37th week that the lung could be considered fully mature and ready to function on its own.
A baby’s first breath is dramatic, and sets off rapid changes: the lungs inflate for the first time and fill air sacs within the lungs, which help keep them from completely deflating when the baby breathes out. Blood flow, no longer needed to carry oxygen from the placenta, is redirected to the lungs and gas exchange begins. The process is quick and probably the most dramatic change for the baby. Think of how much more effort it takes to blow up a brand new balloon compared to one that has been previously inflated. A baby’s first breath will be her most difficult for the rest of her life.