The most profound change at birth is your baby’s first breath. At this point, your baby’s lungs, which were filled with fluid during pregnancy, must suddenly fill with oxygen from the air. The fluid in the lungs is removed through the blood and lymph system, and is replaced by air. Your baby’s lungs must be able to exchange oxygen for carbon dioxide. At the same time, vigorous blood circulation in the lungs will begin. The first few breaths after birth may be the most difficult breaths your baby will take for the rest of her life.
There are a couple of things that will stimulate your baby to take her first breath. Hormonal and other changes during labour slow down or stop the production of fluid in the lungs, and may initiate the reabsorption of fluid from the lungs. This process is unlikely to have occurred if labour was very short or did not occur at all, for example, if your baby was delivered by caesarean section. Furthermore, physical stimulation and handling during delivery will encourage your baby to breathe. There are probably many other factors that stimulate baby’s first breath, but they have not been identified yet.
Your baby will need to work very hard to take her first breath, and her first few breaths may be shallow and irregular. With each breath after birth, more air will accumulate in her lungs, which will make it easier for her to breathe. After a few breaths, your baby will be able to breathe more easily, and her breaths will start to become deeper and more regular.
As the fluid in your baby’s lungs is replaced by air, the increased amounts of oxygen will stimulate a blood vessel close to her heart called the ductus arteriosus to begin closing. The ductus arteriosus was important to your baby’s body before birth, to divert blood away from the lungs. After birth, your baby needs blood to circulate through the lungs, and therefore, the ductus arteriosus is no longer needed. The ductus arteriosus usually closes during the first or second day of life. At this point, your baby’s heart will pump and circulate blood in much the same way as an adult’s heart. The transition from fetal to adult circulation can take minutes or hours. Problems with your baby’s colour or breathing may be related to this delayed transition.