Breathing in the Womb
Babies do not actually breathe in the womb---at least, not in the usual sense. Fetal lungs are not fully functional, and are not even able to fully expand, until after birth. During the later stages of gestation, the fetus may "practice" breathing by inhaling and exhaling amniotic fluid. The fetal lungs do not process the amniotic fluid, the way fully formed lungs process air, but experts believe this "breathing" is important to fetal lung development. The fetus gets all of its oxygen and nutrients through the placenta and umbilical cord---a process called fetal circulation.
Fetal Circulation
In post-natal breathing, blood circulates from the heart, to the lungs, then out to the body. As the blood reaches the lungs, it takes in oxygen and filters out carbon dioxide. In prenatal breathing, or fetal circulation, the oxygen arrives in the mother's blood, via the umbilical cord. The umbilical capillaries in the placenta filter the oxygen from the mother's blood to the fetal blood stream and filters carbon dioxide from the fetal blood to the mother's blood stream. The oxygenated blood flows to the fetal heart but, instead of flowing out to the lungs, the blood remains in the heart and circulates through the fetal body. The fetal heart has a special opening between the right and left atria of the heart called the foramen ovale. The foramen ovale allows blood to bypass the lungs and flow directly to the left atrium and ventricle, and out to the fetal body. The blood then returns to the fetal heart and exits the umbilical vein to the mother's blood stream.
Baby's First Breath
During birth, several processes occur simultaneously---all of which end with the newborn breathing through his lungs. The blood vessels in the umbilical cord constrict, cutting off flow from the placenta. Newborn blood oxygen levels drop rapidly, and carbon dioxide levels rise. The newborn's chest, constricted from the birth canal, expands---causing the lungs to expand. The expansion of the lungs causes the newborn to draw in a breath, which also causes the forum ovale to close. When the forum ovale closes, blood flows to the lungs to pick up oxygen and the lungs fill to capacity with air. In some cases, the doctor may irritate the newborn in order to initiate crying. In other cases, the newborn may cry on her own. In either case, the cry is an important sign that the lungs and pulmonary circuit are working and that the newborn is breathing properly.
