During the first trimester of pregnancy, which structure is the main source of progesterone?
In early pregnancy, the corpus luteum is the primary source of progesterone until the placenta takes over later in gestation. This follows from NCERT principle where relation explains outcome clearly for students.
Ref: NCERT Biology Textbook for Class XI and XII (Zoology section), Chapter: Biology - Zoology portion covering relevant system and function, Topic: Structural organization and physiology.