Your baby's complex lung development is continuing. To picture their growth and development, imagine the whole lungs as a tree: the trunk has developed (the trachea or windpipe), and this has branched into smaller and medium-sized branches (bronchi), but the twigs (bronchioles) holding the leaves (alveoli) have not yet formed. It is the alveoli that have walls so thin that they allow oxygen to be absorbed from the air in them and carbon dioxide to leave via the bloodstream.
From now until 28 weeks the "twigs" are forming that will hold the alveoli that will become filled with air after birth. These "twigs" will have a limited ability to transfer gases but the lungs will not be fully effective until the alveoli develop. The growth of the blood vessels that support the lungs closely matches the growth of the lungs themselves. These blood vessels will be essential for the transfer of oxygen after your baby is born.
After birth, all the blood that is leaving the right side of the heart will enter the lung circulation, but before birth, with the lungs filled with fluid and not used for breathing, only a small amount of blood (approximately 10-15 percent) is directed to them.