At approximately day 1

Normal fetal weight: 1300 gm

Reference

Moore KL, Persaud TVN. 1998. The Developing Human: Clinically Oriented Embryology. 6th ed. Philadelphia: WB Saunders Co.

At approximately day 1

Respiratory bronchioles subdivide to produce terminal sacs (primitive alveoli) numbering between 20 and 70 million before birth.

Duration: 8 weeks

Reference

Larsen WJ. 1997. Human Embryology. New York: Churchill Livingstone Inc.