At approximately day 1

Normal fetal weight: 2900 gm

Reference

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

At approximately day 1

Alveoli mature; new alveoli continue to be formed up to 8 years post-natally.

Duration: 4 weeks

Reference

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