The blood flow to your baby's lungs mirrors the development of the airways. Blood leaves the right side of the heart through a one-way valve into the main pulmonary vein. This then divides to give a pulmonary branch to each lung, and also a duct that allows blood to bypass the lungs and travel to the body directly. This will close soon after birth as the lungs expand and their resistance to blood flow falls.
Because your baby doesn't use his lungs for gas exchange in the uterus, the blood supply to them is quite small-only 10 percent of the post-birth supply. At this stage of pregnancy, the lungs' blood supply has completed its development, branching into finer and finer vessels as they come to lie closer to the alveoli.
When your baby is born, her chest is compressed in the birth canal and this helps to push the fluid out of the lungs in preparation for that incredible first breath. If your baby is born by cesarean, she will need first to bring the fluid up by herself. This is not a problem but for this reason the first breaths of a baby born by cesarean can be full of mucus.