Final answer:
During the latter part of the third trimester of pregnancy, the lungs transform into a gas exchanging organ and the fetus becomes viable.
Step-by-step explanation:
The lungs are transformed into a gas exchanging organ and the fetus becomes viable during the latter part of the third trimester of pregnancy. The major milestone of respiratory development occurs at around week 28, when sufficient alveolar precursors have matured so that a baby born prematurely at this time can usually breathe on its own. The respiratory system continues to develop and mature into childhood, with a full complement of functional alveoli appearing around 8 years of age.