Without Lungs
A fetus in the womb does not begin its existence with fully functioning lungs. Instead, the infant relies on its mother to provide the oxygen required for living. A fetus doesn't really breathe in the womb. The lungs are full of amniotic fluid. Amniotic fluid is an integral part of proper development while baby is in the womb, as it provides the essential nutrients for the lungs and other organs to develop. The mother supplies oxygen to the baby by way of blood.
Hemoglobin
Hemoglobin is a protein found in red blood cells that is responsible for carrying oxygen. While in the womb, infants have more hemoglobin than adults. This allows more oxygen to enter their bodies to help them carry out their normal functions. Oxygen is inhaled by the mother and flows through her blood. Eventually it reaches the placenta and then the umbilical cord. From here, the oxygenated blood cells make their way to the fetus. The cells enter the heart where they are then pumped throughout the fetus' body. To complete the cycle, the blood exits the body through the umbilical cord and back into the mother's blood stream.
Practice Breaths
Somewhere between the second trimester and the final weeks before delivery, an infant begins taking practice breaths. While in the womb, the infant inhales amniotic fluid and exhales. Essentially, the infant is breathing fluids. These practice breaths are thought to prepare the baby for breathing at birth. As an infant is delivered from the womb, a swift transition of relying on the umbilical cord for oxygen to using the lungs takes place. The lungs force the baby to take a breath because the oxygen levels have dropped so greatly at birth.
