Final answer:
Harriet's baby is in the fetal period of prenatal development, a stage characterized by significant growth and development of the respiratory system, allowing the fetus to make breathing movements.
Step-by-step explanation:
Harriet's developing baby is in the fetal period of prenatal development. This stage begins at the end of the embryonic period, specifically from week 9 until birth. During this time, major growth and maturation occurs, including the development of the respiratory system. By around week 24 to term, the fetus is able to make breathing movements and is preparing for life outside the uterus. Although the pregnant person provides oxygen and expels carbon dioxide through the placenta, the fetus practices breathing to prepare for life after birth. By week 28, there are enough mature alveoli for a baby born prematurely to potentially breathe on its own; however, a full complement of functional alveoli is not present until approximately 8 years of age.