After Birth:

• After birth, and during the first couple months
• Lungs continue to grow and expand until it is at its maximum growth.

This diagram depicts what happens in the alveolar phase after birth. More alveoli form from the terminal endings of the alveolar sacculi and increase in diameter overtime. Furthermore, they are also delimited by secondary septa.

http://www.embryology.ch/anglais/rrespiratory/phasen06.html