Final answer:
During the third trimester of pregnancy, the placental formation is already complete.
Step-by-step explanation:
The developmental process that does NOT occur during the third trimester is Placental formation. During the third trimester, the placenta is already formed, and its main function is to provide oxygen and nutrients to the growing fetus. The other processes mentioned, such as the mass doubling, strengthening of bones, and maturation of the lungs, occur during the third trimester.
Learn more about Developmental processes during the third trimester of pregnancy