Stages of lung development, illustration. The lungs develop from around the fourth week of human embryonic development. The lungs become progressively more branched and more able to exchange gases with the blood, with the full-term newborn baby able to breathe unaided. The stages down left are: embryonic (0-7 weeks), pseudoglandular (7-17 weeks), canalicular (16-25 weeks), and saccular (alveolar) from 25 weeks to full term. Over this period, the following structures develop: trachea, the extrapulmonary space, the main bronchus, the bronchi, the bronchioles, the terminal and respiratory bronchioles, the alveolar ducts, and the alveoli. By the time a baby is born, it will have developed 300 to 600 million alveoli (around 1000 per lung acinus or alveolar sac).