At approximately day 1

Normal fetal weight: 200 gm

Reference

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

At approximately day 1

Terminal bronchioles divide into respiratory bronchioles and the respiratory vasculature begins to develop by week 28.

Duration: 12 weeks

Reference

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