Lung development in a fetus starts during the embryonic phase, at around four weeks of gestational age. Very early in development, what will eventually be a baby is nothing more than a cell ball, where none of the cells are yet specialized. An early differentiation event separates cells into several different layers, one of which will become the organs of the gut. It is from these cells that the lungs develop, eventually becoming capable of respiring on their own after a baby is born.
The embryonic phase of lung development takes place during weeks four and five of gestation, notes the University of New South Wales. During this phase, what will become the larynx, or voicebox, and the trachea, or windpipe, form from the foregut. Two small buds branch off, one of which will become the left lung, and one of which will become the right lung.
Until about the 17th week of gestational age, well into the fetal period of development, lungs are in the pseudoglandular phase. The Brown University publication “Introduction to Fetal Medicine” notes that this phase is characterized by further branching of the original lung buds into smaller and more numerous areas. Each bud eventually becomes an independent respiratory unit, served by a bronchiole—a small branch off the trachea—and surrounded by capillary vessels that will bring blood to the lungs for oxygen.
Lasting until approximately 25 weeks of gestational age, the canalicular phase sets up the air-blood barrier that will allow oxygen to enter blood in respiratory capillaries and carbon dioxide to flow out of respiratory capillaries into the lungs for exhalation. The University of Lausanne also notes that this phase is characterized by differentiation of the different tissue types in the lungs, meaning that air transport tissues become distinguishable from gas-exchange tissues.
Until about 36 weeks of gestational age, a fetus is in the saccular phase of lung development. During this time, surfactant production begins in the lungs. Surfactant is a soapy substance that helps keep delicate lung tissue from sticking to itself and tearing during exhalation or if the lungs are compressed. Surfactant is particularly important during delivery, as it allows the lungs to drain of amniotic fluid and fill with air properly. Premature infants are susceptible to respiratory problems and lung collapse if they’re born before sufficient surfactant forms. The University of Lausanne also notes that air sacs begin to fill during this phase.
The last phase of lung development, lasting until birth and beyond into early childhood, is the alveolar phase. The University of New South Wales notes that in addition to additional production of surfactant, lung development during this period is characterized by growth of more bronchioles and air sacs, called alveoli. This allows the gas-exchange tissues of the lungs to expand and makes them capable of moving more air as a baby, and later a child, grows.