In comparison with the development of the other organs the prenatal development of the lungs occupies a special position. The lungs as breathing organs are unnecessary for intrauterine existence. Nevertheless, they must be developed to such an extent that they are immediately ready to function following birth. This explains why the entire development extends from the embryonic period through the fetal period up to birth (and even afterwards).
During intrauterine life, the lungs are nevertheless an important source of amniotic fluid. Daily, ca. 15 ml per kg of body weight is produced that flows out via the trachea and mouth or is swallowed.