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• During the first and second month of gestation, the lungs begin to develop from endoderm and lateral plate mesoderm, and the nasal cavity, upper airways, and thoracic cavity develop. Four phases of lung maturation occur from week 5 and continue through childhood. Premature infants who are born before the terminal phase of lung development (week 24) may have infant respiratory distress syndrome because lung surfactant is not yet secreted in alveoli.

During the third week of gestation, a groove in the primitive foregut originates, and, by week 4, the primordial single lung bud bifurcates.

During weeks 5-6, the primary bronchi continue to elongate and divide, so that by the end of week 6 there are usually 10 segmental bronchi on the right and 8-9 segmental bronchi on the left.

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   By 16 weeks’ gestation, the bronchial tree has developed. By 26–28 weeks’ gestation, sufficient air sacs and pulmonary vasculature have developed so that the fetus is able to survive.
2.  
   Alveolarization begins around 36 weeks’ gestational age with most alveoli forming during the first few years of life, followed by a slower rate later in childhood and continuing through adolescence.¹

Early lung development in a 32-day fetus. From deMello and Reid. Pediat Develop Path. 2000;3:439-449.

References

1. Witcoff, L. J. (2019). Pulmonology. Brown, L. J., Coller, R. J., & Miller, L. T. BRS Pediatrics, 2e. Lippincott Williams & Wilkins, a Wolters Kluwer business. https://brs-lwwhealthlibrary-com.usu01.idm.oclc.org/content.aspx?bookid=2556&sectionid=205035169

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